feat: Improve recurrence invitations messages

Signed-off-by: SebastianKrupinski <krupinskis05@gmail.com>

4 files changed, 1305 insertions, 90 deletions

diff --git a/apps/dav/lib/CalDAV/EventReader.php b/apps/dav/lib/CalDAV/EventReader.php
new file mode 100644
index 00000000000..99e5677d432
--- /dev/null
+++ b/apps/dav/lib/CalDAV/EventReader.php
@@ -0,0 +1,758 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2024 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\DAV\CalDAV;
+
+use DateTime;
+use DateTimeInterface;
+use DateTimeZone;
+use InvalidArgumentException;
+
+use Sabre\VObject\Component\VCalendar;
+use Sabre\VObject\Component\VEvent;
+use Sabre\VObject\Reader;
+
+class EventReader {
+
+	protected VEvent $baseEvent;
+	protected DateTimeInterface $baseEventStartDate;
+	protected DateTimeZone $baseEventStartTimeZone;
+	protected DateTimeInterface $baseEventEndDate;
+	protected DateTimeZone $baseEventEndTimeZone;
+	protected bool $baseEventStartDateFloating = false;
+	protected bool $baseEventEndDateFloating = false;
+	protected int $baseEventDuration;
+
+	protected ?EventReaderRRule $rruleIterator = null;
+	protected ?EventReaderRDate $rdateIterator = null;
+	protected ?EventReaderRRule $eruleIterator = null;
+	protected ?EventReaderRDate $edateIterator = null;
+
+	protected array $recurrenceModified;
+	protected ?DateTimeInterface $recurrenceCurrentDate;
+
+	protected array $dayNamesMap = [
+		'MO' => 'Monday', 'TU' => 'Tuesday', 'WE' => 'Wednesday', 'TH' => 'Thursday', 'FR' => 'Friday', 'SA' => 'Saturday', 'SU' => 'Sunday'
+	];
+	protected array $monthNamesMap = [
+		1 => 'January', 2 => 'February', 3 => 'March', 4 => 'April', 5 => 'May', 6 => 'June',
+		7 => 'July', 8 => 'August', 9 => 'September', 10 => 'October', 11 => 'November', 12 => 'December'
+	];
+	protected array $relativePositionNamesMap = [
+		1 => 'First', 2 => 'Second', 3 => 'Third', 4 => 'Fourth', 5 => 'Fifty',
+		-1 => 'Last', -2 => 'Second Last', -3 => 'Third Last', -4 => 'Fourth Last', -5 => 'Fifty Last'
+	];
+
+	/**
+	 * Initilizes the Event Reader
+	 *
+	 * There is several ways to set up the iterator.
+	 *
+	 * 1. You can pass a VCALENDAR component (as object or string) and a UID.
+	 * 2. You can pass an array of VEVENTs (all UIDS should match).
+	 * 3. You can pass a single VEVENT component (as object or string).
+	 *
+	 * Only the second method is recommended. The other 1 and 3 will be removed
+	 * at some point in the future.
+	 *
+	 * The $uid parameter is only required for the first method.
+	 *
+	 * @since 30.0.0
+	 *
+	 * @param VCalendar|VEvent|Array|String $input
+	 * @param string|null     				$uid
+	 * @param DateTimeZone|null    				$timeZone reference timezone for floating dates and times
+	 */
+	public function __construct(VCalendar|VEvent|array|string $input, ?string $uid = null, ?DateTimeZone $timeZone = null) {
+
+		// evaluate if the input is a string and convert it to and vobject if required
+		if (is_string($input)) {
+			$input = Reader::read($input);
+		}
+		// evaluate if input is a single event vobject and convert it to a collection
+		if ($input instanceof VEvent) {
+			$events = [$input];
+		}
+		// evaluate if input is a calendar vobject
+		elseif ($input instanceof VCalendar) {
+			// Calendar + UID mode.
+			if ($uid === null) {
+				throw new InvalidArgumentException('The UID argument is required when a VCALENDAR object is used');
+			}
+			// extract events from calendar
+			$events = $input->getByUID($uid);
+			// evaluate if any event where found
+			if (count($events) === 0) {
+				throw new InvalidArgumentException('This VCALENDAR did not have an event with UID: '.$uid);
+			}
+			// extract calendar timezone
+			if (isset($input->VTIMEZONE) && isset($input->VTIMEZONE->TZID)) {
+				$calendarTimeZone = new DateTimeZone($input->VTIMEZONE->TZID->getValue());
+			}
+		}
+		// evaluate if input is a collection of event vobjects
+		elseif (is_array($input)) {
+			$events = $input;
+		} else {
+			throw new InvalidArgumentException('Invalid input data type');
+		}
+		// find base event instance and remove it from events collection
+		foreach ($events as $key => $vevent) {
+			if (!isset($vevent->{'RECURRENCE-ID'})) {
+				$this->baseEvent = $vevent;
+				unset($events[$key]);
+			}
+		}
+		
+		// No base event was found. CalDAV does allow cases where only
+		// overridden instances are stored.
+		//
+		// In this particular case, we're just going to grab the first
+		// event and use that instead. This may not always give the
+		// desired result.
+		if (!isset($this->baseEvent) && count($events) > 0) {
+			$this->baseEvent = array_shift($events);
+		}
+
+		// determain the event starting time zone
+		// we require this to align all other dates times
+		// evaluate if timezone paramater was used (treat this as a override)
+		if ($timeZone !== null) {
+			$this->baseEventStartTimeZone = $timeZone;
+		}
+		// evaluate if event start date has a timezone parameter
+		elseif (isset($this->baseEvent->DTSTART->parameters['TZID'])) {
+			$this->baseEventStartTimeZone = new DateTimeZone($this->baseEvent->DTSTART->parameters['TZID']->getValue());
+		}
+		// evaluate if event parent calendar has a time zone
+		elseif (isset($calendarTimeZone)) {
+			$this->baseEventStartTimeZone = clone $calendarTimeZone;
+		}
+		// otherwise, as a last resort use the UTC timezone
+		else {
+			$this->baseEventStartTimeZone = new DateTimeZone('UTC');
+		}
+
+		// determain the event end time zone
+		// we require this to align all other dates and times
+		// evaluate if timezone paramater was used (treat this as a override)
+		if ($timeZone !== null) {
+			$this->baseEventEndTimeZone = $timeZone;
+		}
+		// evaluate if event end date has a timezone parameter
+		elseif (isset($this->baseEvent->DTEND->parameters['TZID'])) {
+			$this->baseEventEndTimeZone = new DateTimeZone($this->baseEvent->DTEND->parameters['TZID']->getValue());
+		}
+		// evaluate if event parent calendar has a time zone
+		elseif (isset($calendarTimeZone)) {
+			$this->baseEventEndTimeZone = clone $calendarTimeZone;
+		}
+		// otherwise, as a last resort use the start date time zone
+		else {
+			$this->baseEventEndTimeZone = clone $this->baseEventStartTimeZone;
+		}
+		// extract start date and time
+		$this->baseEventStartDate = $this->baseEvent->DTSTART->getDateTime($this->baseEventStartTimeZone);
+		$this->baseEventStartDateFloating = $this->baseEvent->DTSTART->isFloating();
+		// determine event end date and duration
+		// evaluate if end date exists
+		// extract end date and calculate duration
+		if (isset($this->baseEvent->DTEND)) {
+			$this->baseEventEndDate = $this->baseEvent->DTEND->getDateTime($this->baseEventEndTimeZone);
+			$this->baseEventEndDateFloating = $this->baseEvent->DTEND->isFloating();
+			$this->baseEventDuration =
+				$this->baseEvent->DTEND->getDateTime($this->baseEventEndTimeZone)->getTimeStamp() -
+				$this->baseEventStartDate->getTimeStamp();
+		}
+		// evaluate if duration exists
+		// extract duration and calculate end date
+		elseif (isset($this->baseEvent->DURATION)) {
+			$this->baseEventDuration = $this->baseEvent->DURATION->getDateInterval();
+			$this->baseEventEndDate = ((clone $this->baseEventStartDate)->add($this->baseEventDuration));
+		}
+		// evaluate if start date is floating
+		// set duration to 24 hours and calculate the end date
+		// according to the rfc any event without a end date or duration is a complete day
+		elseif ($this->baseEventStartDateFloating == true) {
+			$this->baseEventDuration = 86400;
+			$this->baseEventEndDate = ((clone $this->baseEventStartDate)->add($this->baseEventDuration));
+		}
+		// otherwise, set duration to zero this should never happen
+		else {
+			$this->baseEventDuration = 0;
+			$this->baseEventEndDate = $this->baseEventStartDate;
+		}
+		// evaluate if RRULE exist and construct iterator
+		if (isset($this->baseEvent->RRULE)) {
+			$this->rruleIterator = new EventReaderRRule(
+				$this->baseEvent->RRULE->getParts(),
+				$this->baseEventStartDate
+			);
+		}
+		// evaluate if RDATE exist and construct iterator
+		if (isset($this->baseEvent->RDATE)) {
+			$this->rdateIterator = new EventReaderRDate(
+				$this->baseEvent->RDATE->getValue(),
+				$this->baseEventStartDate
+			);
+		}
+		// evaluate if EXRULE exist and construct iterator
+		if (isset($this->baseEvent->EXRULE)) {
+			$this->eruleIterator = new EventReaderRRule(
+				$this->baseEvent->EXRULE->getParts(),
+				$this->baseEventStartDate
+			);
+		}
+		// evaluate if EXDATE exist and construct iterator
+		if (isset($this->baseEvent->EXDATE)) {
+			$this->edateIterator = new EventReaderRDate(
+				$this->baseEvent->EXDATE->getValue(),
+				$this->baseEventStartDate
+			);
+		}
+		// construct collection of modified events with recurrence id as hash
+		foreach ($events as $vevent) {
+			$this->recurrenceModified[$vevent->{'RECURRENCE-ID'}->getDateTime($this->baseEventStartTimeZone)->getTimeStamp()] = $vevent;
+		}
+		
+		$this->recurrenceCurrentDate = clone $this->baseEventStartDate;
+	}
+
+	/**
+	 * retrieve date and time of event start
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return DateTime
+	 */
+	public function startDateTime(): DateTime {
+		return DateTime::createFromInterface($this->baseEventStartDate);
+	}
+
+	/**
+	 * retrieve time zone of event start
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return DateTimeZone
+	 */
+	public function startTimeZone(): DateTimeZone {
+		return $this->baseEventStartTimeZone;
+	}
+
+	/**
+	 * retrieve date and time of event end
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return DateTime
+	 */
+	public function endDateTime(): DateTime {
+		return DateTime::createFromInterface($this->baseEventEndDate);
+	}
+
+	/**
+	 * retrieve time zone of event end
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return DateTimeZone
+	 */
+	public function endTimeZone(): DateTimeZone {
+		return $this->baseEventEndTimeZone;
+	}
+
+	/**
+	 * is this an all day event
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return bool
+	 */
+	public function entireDay(): bool {
+		return $this->baseEventStartDateFloating;
+	}
+
+	/**
+	 * is this a recurring event
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return bool
+	 */
+	public function recurs(): bool {
+		return ($this->rruleIterator !== null || $this->rdateIterator !== null);
+	}
+
+	/**
+	 * event recurrence pattern
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return string|null				R - Relative or A - Absolute
+	 */
+	public function recurringPattern(): string | null {
+		if ($this->rruleIterator === null && $this->rdateIterator === null) {
+			return null;
+		}
+		if ($this->rruleIterator?->isRelative()) {
+			return 'R';
+		}
+		return 'A';
+	}
+
+	/**
+	 * event recurrence precision
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return string|null			daily, weekly, monthly, yearly, fixed
+	 */
+	public function recurringPrecision(): string | null {
+		if ($this->rruleIterator !== null) {
+			return $this->rruleIterator->precision();
+		}
+		if ($this->rdateIterator !== null) {
+			return 'fixed';
+		}
+		return null;
+	}
+
+	/**
+	 * event recurrence interval
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return int|null
+	 */
+	public function recurringInterval(): int | null {
+		return $this->rruleIterator?->interval();
+	}
+
+	/**
+	 * event recurrence conclusion
+	 *
+	 * returns true if RRULE with UNTIL or COUNT (calculated) is used
+	 * returns true RDATE is used
+	 * returns false if RRULE or RDATE are absent, or RRRULE is infinite
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return bool
+	 */
+	public function recurringConcludes(): bool {
+
+		// retrieve rrule conclusions
+		if ($this->rruleIterator?->concludesOn() !== null ||
+			$this->rruleIterator?->concludesAfter() !== null) {
+			return true;
+		}
+		// retrieve rdate conclusions
+		if ($this->rdateIterator?->concludesAfter() !== null) {
+			return true;
+		}
+
+		return false;
+
+	}
+
+	/**
+	 * event recurrence conclusion iterations
+	 *
+	 * returns the COUNT value if RRULE is used
+	 * returns the collection count if RDATE is used
+	 * returns combined count of RRULE COUNT and RDATE if both are used
+	 * returns null if RRULE and RDATE are absent
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return int|null
+	 */
+	public function recurringConcludesAfter(): int | null {
+		
+		// construct count place holder
+		$count = 0;
+		// retrieve and add RRULE iterations count
+		$count += (int) $this->rruleIterator?->concludesAfter();
+		// retrieve and add RDATE iterations count
+		$count += (int) $this->rdateIterator?->concludesAfter();
+		// return count
+		return !empty($count) ? $count : null;
+
+	}
+
+	/**
+	 * event recurrence conclusion date
+	 *
+	 * returns the last date of UNTIL or COUNT (calculated) if RRULE is used
+	 * returns the last date in the collection if RDATE is used
+	 * returns the highest date if both RRULE and RDATE are used
+	 * returns null if RRULE and RDATE are absent or RRULE is infinite
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return DateTime|null
+	 */
+	public function recurringConcludesOn(): DateTime | null {
+
+		if ($this->rruleIterator !== null) {
+			// retrieve rrule conclusion date
+			$rrule = $this->rruleIterator->concludes();
+			// evaluate if rrule conclusion is null
+			// if this is null that means the recurrence is infinate
+			if ($rrule === null) {
+				return null;
+			}
+		}
+		// retrieve rdate conclusion date
+		if ($this->rdateIterator !== null) {
+			$rdate = $this->rdateIterator->concludes();
+		}
+		// evaluate if both rrule and rdate have date
+		if (isset($rdate) && isset($rrule)) {
+			// return the highest date
+			return (($rdate > $rrule) ? $rdate : $rrule);
+		} elseif (isset($rrule)) {
+			return $rrule;
+		} elseif (isset($rdate)) {
+			return $rdate;
+		}
+
+		return null;
+
+	}
+
+	/**
+	 * event recurrence days of the week
+	 *
+	 * returns collection of RRULE BYDAY day(s) ['MO','WE','FR']
+	 * returns blank collection if RRULE is absent, RDATE presents or absents has no affect
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return array
+	 */
+	public function recurringDaysOfWeek(): array {
+		// evaluate if RRULE exists and return day(s) of the week
+		return $this->rruleIterator !== null ? $this->rruleIterator->daysOfWeek() : [];
+	}
+
+	/**
+	 * event recurrence days of the week (named)
+	 *
+	 * returns collection of RRULE BYDAY day(s) ['Monday','Wednesday','Friday']
+	 * returns blank collection if RRULE is absent, RDATE presents or absents has no affect
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return array
+	 */
+	public function recurringDaysOfWeekNamed(): array {
+		// evaluate if RRULE exists and extract day(s) of the week
+		$days = $this->rruleIterator !== null ? $this->rruleIterator->daysOfWeek() : [];
+		// convert numberic month to month name
+		foreach ($days as $key => $value) {
+			$days[$key] = $this->dayNamesMap[$value];
+		}
+		// return names collection
+		return $days;
+	}
+
+	/**
+	 * event recurrence days of the month
+	 *
+	 * returns collection of RRULE BYMONTHDAY day(s) [7, 15, 31]
+	 * returns blank collection if RRULE is absent, RDATE presents or absents has no affect
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return array
+	 */
+	public function recurringDaysOfMonth(): array {
+		// evaluate if RRULE exists and return day(s) of the month
+		return $this->rruleIterator !== null ? $this->rruleIterator->daysOfMonth() : [];
+	}
+
+	/**
+	 * event recurrence days of the year
+	 *
+	 * returns collection of RRULE BYYEARDAY day(s) [57, 205, 365]
+	 * returns blank collection if RRULE is absent, RDATE presents or absents has no affect
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return array
+	 */
+	public function recurringDaysOfYear(): array {
+		// evaluate if RRULE exists and return day(s) of the year
+		return $this->rruleIterator !== null ? $this->rruleIterator->daysOfYear() : [];
+	}
+
+	/**
+	 * event recurrence weeks of the month
+	 *
+	 * returns collection of RRULE SETPOS weeks(s) [1, 3, -1]
+	 * returns blank collection if RRULE is absent or SETPOS is absent, RDATE presents or absents has no affect
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return array
+	 */
+	public function recurringWeeksOfMonth(): array {
+		// evaluate if RRULE exists and RRULE is relative return relative position(s)
+		return $this->rruleIterator?->isRelative() ? $this->rruleIterator->relativePosition() : [];
+	}
+
+	/**
+	 * event recurrence weeks of the month (named)
+	 *
+	 * returns collection of RRULE SETPOS weeks(s) [1, 3, -1]
+	 * returns blank collection if RRULE is absent or SETPOS is absent, RDATE presents or absents has no affect
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return array
+	 */
+	public function recurringWeeksOfMonthNamed(): array {
+		// evaluate if RRULE exists and extract relative position(s)
+		$positions = $this->rruleIterator?->isRelative() ? $this->rruleIterator->relativePosition() : [];
+		// convert numberic relative position to relative label
+		foreach ($positions as $key => $value) {
+			$positions[$key] = $this->relativePositionNamesMap[$value];
+		}
+		// return positions collection
+		return $positions;
+	}
+
+	/**
+	 * event recurrence weeks of the year
+	 *
+	 * returns collection of RRULE BYWEEKNO weeks(s) [12, 32, 52]
+	 * returns blank collection if RRULE is absent, RDATE presents or absents has no affect
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return array
+	 */
+	public function recurringWeeksOfYear(): array {
+		// evaluate if RRULE exists and return weeks(s) of the year
+		return $this->rruleIterator !== null ? $this->rruleIterator->weeksOfYear() : [];
+	}
+
+	/**
+	 * event recurrence months of the year
+	 *
+	 * returns collection of RRULE BYMONTH month(s) [3, 7, 12]
+	 * returns blank collection if RRULE is absent, RDATE presents or absents has no affect
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return array
+	 */
+	public function recurringMonthsOfYear(): array {
+		// evaluate if RRULE exists and return month(s) of the year
+		return $this->rruleIterator !== null ? $this->rruleIterator->monthsOfYear() : [];
+	}
+
+	/**
+	 * event recurrence months of the year (named)
+	 *
+	 * returns collection of RRULE BYMONTH month(s) [3, 7, 12]
+	 * returns blank collection if RRULE is absent, RDATE presents or absents has no affect
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return array
+	 */
+	public function recurringMonthsOfYearNamed(): array {
+		// evaluate if RRULE exists and extract month(s) of the year
+		$months = $this->rruleIterator !== null ? $this->rruleIterator->monthsOfYear() : [];
+		// convert numberic month to month name
+		foreach ($months as $key => $value) {
+			$months[$key] = $this->monthNamesMap[$value];
+		}
+		// return months collection
+		return $months;
+	}
+
+	/**
+	 * event recurrence relative positions
+	 *
+	 * returns collection of RRULE SETPOS value(s) [1, 5, -3]
+	 * returns blank collection if RRULE is absent, RDATE presents or absents has no affect
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return array
+	 */
+	public function recurringRelativePosition(): array {
+		// evaluate if RRULE exists and return relative position(s)
+		return $this->rruleIterator !== null ? $this->rruleIterator->relativePosition() : [];
+	}
+
+	/**
+	 * event recurrence relative positions (named)
+	 *
+	 * returns collection of RRULE SETPOS [1, 3, -1]
+	 * returns blank collection if RRULE is absent or SETPOS is absent, RDATE presents or absents has no affect
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return array
+	 */
+	public function recurringRelativePositionNamed(): array {
+		// evaluate if RRULE exists and extract relative position(s)
+		$positions = $this->rruleIterator?->isRelative() ? $this->rruleIterator->relativePosition() : [];
+		// convert numberic relative position to relative label
+		foreach ($positions as $key => $value) {
+			$positions[$key] = $this->relativePositionNamesMap[$value];
+		}
+		// return positions collection
+		return $positions;
+	}
+
+	/**
+	 * event recurrence date
+	 *
+	 * returns date of currently selected recurrence
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return DateTime
+	 */
+	public function recurrenceDate(): DateTime | null {
+		if ($this->recurrenceCurrentDate !== null) {
+			return DateTime::createFromInterface($this->recurrenceCurrentDate);
+		} else {
+			return null;
+		}
+	}
+
+	/**
+	 * event recurrence rewind
+	 *
+	 * sets the current recurrence to the first recurrence in the collection
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return void
+	 */
+	public function recurrenceRewind(): void {
+		// rewind and increment rrule
+		if ($this->rruleIterator !== null) {
+			$this->rruleIterator->rewind();
+		}
+		// rewind and increment rdate
+		if ($this->rdateIterator !== null) {
+			$this->rdateIterator->rewind();
+		}
+		// rewind and increment exrule
+		if ($this->eruleIterator !== null) {
+			$this->eruleIterator->rewind();
+		}
+		// rewind and increment exdate
+		if ($this->edateIterator !== null) {
+			$this->edateIterator->rewind();
+		}
+		// set current date to event start date
+		$this->recurrenceCurrentDate = clone $this->baseEventStartDate;
+	}
+
+	/**
+	 * event recurrence advance
+	 *
+	 * sets the current recurrence to the next recurrence in the collection
+	 *
+	 * @since 30.0.0
+	 *
+	 * @return void
+	 */
+	public function recurrenceAdvance(): void {
+		// place holders
+		$nextOccurrenceDate = null;
+		$nextExceptionDate = null;
+		$rruleDate = null;
+		$rdateDate = null;
+		$eruleDate = null;
+		$edateDate = null;
+		// evaludate if rrule is set and advance one interation past current date
+		if ($this->rruleIterator !== null) {
+			// forward rrule to the next future date
+			while ($this->rruleIterator->valid() && $this->rruleIterator->current() <= $this->recurrenceCurrentDate) {
+				$this->rruleIterator->next();
+			}
+			$rruleDate = $this->rruleIterator->current();
+		}
+		// evaludate if rdate is set and advance one interation past current date
+		if ($this->rdateIterator !== null) {
+			// forward rdate to the next future date
+			while ($this->rdateIterator->valid() && $this->rdateIterator->current() <= $this->recurrenceCurrentDate) {
+				$this->rdateIterator->next();
+			}
+			$rdateDate = $this->rdateIterator->current();
+		}
+		if ($rruleDate !== null && $rdateDate !== null) {
+			$nextOccurrenceDate = ($rruleDate <= $rdateDate) ? $rruleDate : $rdateDate;
+		} elseif ($rruleDate !== null) {
+			$nextOccurrenceDate = $rruleDate;
+		} elseif ($rdateDate !== null) {
+			$nextOccurrenceDate = $rdateDate;
+		}
+
+		// evaludate if exrule is set and advance one interation past current date
+		if ($this->eruleIterator !== null) {
+			// forward exrule to the next future date
+			while ($this->eruleIterator->valid() && $this->eruleIterator->current() <= $this->recurrenceCurrentDate) {
+				$this->eruleIterator->next();
+			}
+			$eruleDate = $this->eruleIterator->current();
+		}
+		// evaludate if exdate is set and advance one interation past current date
+		if ($this->edateIterator !== null) {
+			// forward exdate to the next future date
+			while ($this->edateIterator->valid() && $this->edateIterator->current() <= $this->recurrenceCurrentDate) {
+				$this->edateIterator->next();
+			}
+			$edateDate = $this->edateIterator->current();
+		}
+		// evaludate if exrule and exdate are set and set nextExDate to the first next date
+		if ($eruleDate !== null && $edateDate !== null) {
+			$nextExceptionDate = ($eruleDate <= $edateDate) ? $eruleDate : $edateDate;
+		} elseif ($eruleDate !== null) {
+			$nextExceptionDate = $eruleDate;
+		} elseif ($edateDate !== null) {
+			$nextExceptionDate = $edateDate;
+		}
+		// if the next date is part of exrule or exdate find another date
+		if ($nextOccurrenceDate !== null && $nextExceptionDate !== null && $nextOccurrenceDate == $nextExceptionDate) {
+			$this->recurrenceCurrentDate = $nextOccurrenceDate;
+			$this->recurrenceAdvance();
+		} else {
+			$this->recurrenceCurrentDate = $nextOccurrenceDate;
+		}
+	}
+
+	/**
+	 * event recurrence advance
+	 *
+	 * sets the current recurrence to the next recurrence in the collection after the specific date
+	 *
+	 * @since 30.0.0
+	 *
+	 * @param DateTimeInterface $dt			date and time to advance
+	 *
+	 * @return void
+	 */
+	public function recurrenceAdvanceTo(DateTimeInterface $dt): void {
+		while ($this->recurrenceCurrentDate !== null && $this->recurrenceCurrentDate < $dt) {
+			$this->recurrenceAdvance();
+		}
+	}
+
+}
diff --git a/apps/dav/lib/CalDAV/EventReaderRDate.php b/apps/dav/lib/CalDAV/EventReaderRDate.php
new file mode 100644
index 00000000000..65362be4b07
--- /dev/null
+++ b/apps/dav/lib/CalDAV/EventReaderRDate.php
@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2024 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\DAV\CalDAV;
+
+use DateTime;
+
+class EventReaderRDate extends \Sabre\VObject\Recur\RDateIterator {
+
+	public function concludes(): DateTime | null {
+		return $this->concludesOn();
+	}
+
+	public function concludesAfter(): int | null {
+		return !empty($this->dates) ? count($this->dates) : null;
+	}
+
+	public function concludesOn(): DateTime | null {
+		if (count($this->dates) > 0) {
+			return new DateTime(
+				$this->dates[array_key_last($this->dates)],
+				$this->startDate->getTimezone()
+			);
+		}
+
+		return null;
+	}
+
+}
diff --git a/apps/dav/lib/CalDAV/EventReaderRRule.php b/apps/dav/lib/CalDAV/EventReaderRRule.php
new file mode 100644
index 00000000000..965abb4c9cd
--- /dev/null
+++ b/apps/dav/lib/CalDAV/EventReaderRRule.php
@@ -0,0 +1,87 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2024 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\DAV\CalDAV;
+
+use DateTime;
+use DateTimeInterface;
+
+class EventReaderRRule extends \Sabre\VObject\Recur\RRuleIterator {
+
+	public function precision(): string {
+		return $this->frequency;
+	}
+
+	public function interval(): int {
+		return $this->interval;
+	}
+
+	public function concludes(): DateTime | null {
+		// evaluate if until value is a date
+		if ($this->until instanceof DateTimeInterface) {
+			return DateTime::createFromInterface($this->until);
+		}
+		// evaluate if count value is higher than 0
+		if ($this->count > 0) {
+			// temporarily store current recurrence date and counter
+			$currentReccuranceDate = $this->currentDate;
+			$currentCounter = $this->counter;
+			// iterate over occurrences until last one (subtract 2 from count for start and end occurrence)
+			while ($this->counter <= ($this->count - 2)) {
+				$this->next();
+			}
+			// temporarly store last reccurance date
+			$lastReccuranceDate = $this->currentDate;
+			// restore current recurrence date and counter
+			$this->currentDate = $currentReccuranceDate;
+			$this->counter = $currentCounter;
+			// return last recurrence date
+			return DateTime::createFromInterface($lastReccuranceDate);
+		}
+
+		return null;
+	}
+
+	public function concludesAfter(): int | null {
+		return !empty($this->count) ? $this->count : null;
+	}
+
+	public function concludesOn(): DateTime | null {
+		return isset($this->until) ? DateTime::createFromInterface($this->until) : null;
+	}
+
+	public function daysOfWeek(): array {
+		return $this->byDay;
+	}
+
+	public function daysOfMonth(): array {
+		return $this->byMonthDay;
+	}
+
+	public function daysOfYear(): array {
+		return $this->byYearDay;
+	}
+
+	public function weeksOfYear(): array {
+		return $this->byWeekNo;
+	}
+
+	public function monthsOfYear(): array {
+		return $this->byMonth;
+	}
+
+	public function isRelative(): bool {
+		return isset($this->bySetPos);
+	}
+
+	public function relativePosition(): array {
+		return $this->bySetPos;
+	}
+
+}
diff --git a/apps/dav/lib/CalDAV/Schedule/IMipService.php b/apps/dav/lib/CalDAV/Schedule/IMipService.php
index 5141f519588..d3ad9a79254 100644
--- a/apps/dav/lib/CalDAV/Schedule/IMipService.php
+++ b/apps/dav/lib/CalDAV/Schedule/IMipService.php
@@ -9,6 +9,8 @@ declare(strict_types=1);
 namespace OCA\DAV\CalDAV\Schedule;
 
 use OC\URLGenerator;
+use OCA\DAV\CalDAV\EventReader;
+use OCP\AppFramework\Utility\ITimeFactory;
 use OCP\IConfig;
 use OCP\IDBConnection;
 use OCP\IL10N;
@@ -31,6 +33,7 @@ class IMipService {
 	private ISecureRandom $random;
 	private L10NFactory $l10nFactory;
 	private IL10N $l10n;
+	private ITimeFactory $timeFactory;
 
 	/** @var string[] */
 	private const STRING_DIFF = [
@@ -44,7 +47,8 @@ class IMipService {
 		IConfig $config,
 		IDBConnection $db,
 		ISecureRandom $random,
-		L10NFactory $l10nFactory) {
+		L10NFactory $l10nFactory,
+		ITimeFactory $timeFactory) {
 		$this->urlGenerator = $urlGenerator;
 		$this->config = $config;
 		$this->db = $db;
@@ -52,6 +56,7 @@ class IMipService {
 		$this->l10nFactory = $l10nFactory;
 		$default = $this->l10nFactory->findGenericLanguage();
 		$this->l10n = $this->l10nFactory->get('dav', $default);
+		$this->timeFactory = $timeFactory;
 	}
 
 	/**
@@ -130,9 +135,13 @@ class IMipService {
 	 * @return array
 	 */
 	public function buildBodyData(VEvent $vEvent, ?VEvent $oldVEvent): array {
+
+		// construct event reader
+		$eventReaderCurrent = new EventReader($vEvent);
+		$eventReaderPrevious = !empty($oldVEvent) ? new EventReader($oldVEvent) : null;
 		$defaultVal = '';
 		$data = [];
-		$data['meeting_when'] = $this->generateWhenString($vEvent);
+		$data['meeting_when'] = $this->generateWhenString($eventReaderCurrent);
 
 		foreach(self::STRING_DIFF as $key => $property) {
 			$data[$key] = self::readPropertyWithDefault($vEvent, $property, $defaultVal);
@@ -145,7 +154,7 @@ class IMipService {
 		}
 
 		if(!empty($oldVEvent)) {
-			$oldMeetingWhen = $this->generateWhenString($oldVEvent);
+			$oldMeetingWhen = $this->generateWhenString($eventReaderPrevious);
 			$data['meeting_title_html'] = $this->generateDiffString($vEvent, $oldVEvent, 'SUMMARY', $data['meeting_title']);
 			$data['meeting_description_html'] = $this->generateDiffString($vEvent, $oldVEvent, 'DESCRIPTION', $data['meeting_description']);
 			$data['meeting_location_html'] = $this->generateLinkifiedDiffString($vEvent, $oldVEvent, 'LOCATION', $data['meeting_location']);
@@ -153,107 +162,334 @@ class IMipService {
 			$oldUrl = self::readPropertyWithDefault($oldVEvent, 'URL', $defaultVal);
 			$data['meeting_url_html'] = !empty($oldUrl) && $oldUrl !== $data['meeting_url'] ? sprintf('<a href="%1$s">%1$s</a>', $oldUrl) : $data['meeting_url'];
 
-			$data['meeting_when_html'] =
-				($oldMeetingWhen !== $data['meeting_when'] && $oldMeetingWhen !== null)
-					? sprintf("<span style='text-decoration: line-through'>%s</span><br />%s", $oldMeetingWhen, $data['meeting_when'])
-					: $data['meeting_when'];
+			$data['meeting_when_html'] = $oldMeetingWhen !== $data['meeting_when'] ? sprintf("<span style='text-decoration: line-through'>%s</span><br />%s", $oldMeetingWhen, $data['meeting_when']) : $data['meeting_when'];
+		}
+		// generate occuring next string
+		if ($eventReaderCurrent->recurs()) {
+			$data['meeting_occurring'] = $this->generateOccurringString($eventReaderCurrent);
 		}
+		
 		return $data;
 	}
 
 	/**
-	 * @param IL10N $this->l10n
-	 * @param VEvent $vevent
-	 * @return false|int|string
+	 * genarates a when string based on if a event has an recurrence or not
+	 *
+	 * @since 30.0.0
+	 *
+	 * @param EventReader $er
+	 *
+	 * @return string
 	 */
-	public function generateWhenString(VEvent $vevent) {
-		/** @var Property\ICalendar\DateTime $dtstart */
-		$dtstart = $vevent->DTSTART;
-		if (isset($vevent->DTEND)) {
-			/** @var Property\ICalendar\DateTime $dtend */
-			$dtend = $vevent->DTEND;
-		} elseif (isset($vevent->DURATION)) {
-			$isFloating = $dtstart->isFloating();
-			$dtend = clone $dtstart;
-			$endDateTime = $dtend->getDateTime();
-			$endDateTime = $endDateTime->add(DateTimeParser::parse($vevent->DURATION->getValue()));
-			$dtend->setDateTime($endDateTime, $isFloating);
-		} elseif (!$dtstart->hasTime()) {
-			$isFloating = $dtstart->isFloating();
-			$dtend = clone $dtstart;
-			$endDateTime = $dtend->getDateTime();
-			$endDateTime = $endDateTime->modify('+1 day');
-			$dtend->setDateTime($endDateTime, $isFloating);
-		} else {
-			$dtend = clone $dtstart;
-		}
-
-		/** @var Property\ICalendar\Date | Property\ICalendar\DateTime $dtstart */
-		/** @var \DateTimeImmutable $dtstartDt */
-		$dtstartDt = $dtstart->getDateTime();
+	public function generateWhenString(EventReader $er): string {
+		return match ($er->recurs()) {
+			true => $this->generateWhenStringRecurring($er),
+			false => $this->generateWhenStringSingular($er)
+		};
+	}
 
-		/** @var Property\ICalendar\Date | Property\ICalendar\DateTime $dtend */
-		/** @var \DateTimeImmutable $dtendDt */
-		$dtendDt = $dtend->getDateTime();
+	/**
+	 * genarates a when string for a non recurring event
+	 *
+	 * @since 30.0.0
+	 *
+	 * @param EventReader $er
+	 *
+	 * @return string
+	 */
+	public function generateWhenStringSingular(EventReader $er): string {
+		// calculate time differnce from now to start of event
+		$occuring = $this->minimizeInterval($this->timeFactory->getDateTime()->diff($er->recurrenceDate()));
+		// extract start date
+		$startDate = $this->l10n->l('date', $er->startDateTime(), ['width' => 'full']);
+		// time of the day
+		if (!$er->entireDay()) {
+			$startTime = $this->l10n->l('time', $er->startDateTime(), ['width' => 'short']);
+			$startTime .= $er->startTimeZone() != $er->endTimeZone() ? ' (' . $er->startTimeZone()->getName() . ')' : '';
+			$endTime = $this->l10n->l('time', $er->endDateTime(), ['width' => 'short']) . ' (' . $er->endTimeZone()->getName() . ')';
+		}
+		// generate localized when string
+		return match ([($occuring[0] > 1), !empty($endTime)]) {
+			[false, false] => $this->l10n->t('In a %1$s on %2$s for the entire day', [$occuring[1], $startDate]),
+			[false, true] => $this->l10n->t('In a %1$s on %2$s between %3$s - %4$s', [$occuring[1], $startDate, $startTime, $endTime]),
+			[true, false] => $this->l10n->t('In %1$s %2$s on %3$s for the entire day', [$occuring[0], $occuring[1], $startDate]),
+			[true, true] => $this->l10n->t('In %1$s %2$s on %3$s between %4$s - %5$s', [$occuring[0], $occuring[1], $startDate, $startTime, $endTime]),
+			default => $this->l10n->t('Could not generate when statement')
+		};
+	}
 
-		$diff = $dtstartDt->diff($dtendDt);
+	/**
+	 * genarates a when string based on recurrance precision/frequency
+	 *
+	 * @since 30.0.0
+	 *
+	 * @param EventReader $er
+	 *
+	 * @return string
+	 */
+	public function generateWhenStringRecurring(EventReader $er): string {
+		return match ($er->recurringPrecision()) {
+			'daily' => $this->generateWhenStringRecurringDaily($er),
+			'weekly' => $this->generateWhenStringRecurringWeekly($er),
+			'monthly' => $this->generateWhenStringRecurringMonthly($er),
+			'yearly' => $this->generateWhenStringRecurringYearly($er),
+			'fixed' => $this->generateWhenStringRecurringFixed($er),
+		};
+	}
 
-		$dtstartDt = new \DateTime($dtstartDt->format(\DateTimeInterface::ATOM));
-		$dtendDt = new \DateTime($dtendDt->format(\DateTimeInterface::ATOM));
+	/**
+	 * genarates a when string for a daily precision/frequency
+	 *
+	 * @since 30.0.0
+	 *
+	 * @param EventReader $er
+	 *
+	 * @return string
+	 */
+	public function generateWhenStringRecurringDaily(EventReader $er): string {
+		
+		// initialize
+		$interval = (int) $er->recurringInterval();
+		$startTime = '';
+		$endTime = '';
+		$conclusion = '';
+		// time of the day
+		if (!$er->entireDay()) {
+			$startTime = $this->l10n->l('time', $er->startDateTime(), ['width' => 'short']);
+			$startTime .= $er->startTimeZone() != $er->endTimeZone() ? ' (' . $er->startTimeZone()->getName() . ')' : '';
+			$endTime = $this->l10n->l('time', $er->endDateTime(), ['width' => 'short']) . ' (' . $er->endTimeZone()->getName() . ')';
+		}
+		// conclusion
+		if ($er->recurringConcludes()) {
+			$conclusion = $this->l10n->l('date', $er->recurringConcludesOn(), ['width' => 'long']);
+		}
+		// generate localized when string
+		return match ([($interval > 1), !empty($startTime), !empty($conclusion)]) {
+			[false, false, false] => $this->l10n->t('Every Day for the entire day'),
+			[false, false, true] => $this->l10n->t('Every Day for the entire day until %1$s', [$conclusion]),
+			[false, true, false] => $this->l10n->t('Every Day between %1$s - %2$s', [$startTime, $endTime]),
+			[false, true, true] => $this->l10n->t('Every Day between %1$s - %2$s until %3$s', [$startTime, $endTime, $conclusion]),
+			[true, false, false] => $this->l10n->t('Every %1$d Days for the entire day', [$interval]),
+			[true, false, true] => $this->l10n->t('Every %1$d Days for the entire day until %2$s', [$interval, $conclusion]),
+			[true, true, false] => $this->l10n->t('Every %1$d Days between %2$s - %3$s', [$interval, $startTime, $endTime]),
+			[true, true, true] => $this->l10n->t('Every %1$d Days between %2$s - %3$s until %4$s', [$interval, $startTime, $endTime, $conclusion]),
+			default => $this->l10n->t('Could not generate event recurrence statement')
+		};
 
-		if ($dtstart instanceof Property\ICalendar\Date) {
-			// One day event
-			if ($diff->days === 1) {
-				return $this->l10n->l('date', $dtstartDt, ['width' => 'medium']);
-			}
+	}
 
-			// DTEND is exclusive, so if the ics data says 2020-01-01 to 2020-01-05,
-			// the email should show 2020-01-01 to 2020-01-04.
-			$dtendDt->modify('-1 day');
+	/**
+	 * genarates a when string for a weekly precision/frequency
+	 *
+	 * @since 30.0.0
+	 *
+	 * @param EventReader $er
+	 *
+	 * @return string
+	 */
+	public function generateWhenStringRecurringWeekly(EventReader $er): string {
+		
+		// initialize
+		$interval = (int) $er->recurringInterval();
+		$startTime = '';
+		$endTime = '';
+		$conclusion = '';
+		// days of the week
+		$days = implode(', ', array_map(function ($value) { return $this->localizeDayName($value); }, $er->recurringDaysOfWeekNamed()));
+		// time of the day
+		if (!$er->entireDay()) {
+			$startTime = $this->l10n->l('time', $er->startDateTime(), ['width' => 'short']);
+			$startTime .= $er->startTimeZone() != $er->endTimeZone() ? ' (' . $er->startTimeZone()->getName() . ')' : '';
+			$endTime = $this->l10n->l('time', $er->endDateTime(), ['width' => 'short']) . ' (' . $er->endTimeZone()->getName() . ')';
+		}
+		// conclusion
+		if ($er->recurringConcludes()) {
+			$conclusion = $this->l10n->l('date', $er->recurringConcludesOn(), ['width' => 'long']);
+		}
+		// generate localized when string
+		return match ([($interval > 1), !empty($startTime), !empty($conclusion)]) {
+			[false, false, false] => $this->l10n->t('Every Week on %1$s for the entire day', [$days]),
+			[false, false, true] => $this->l10n->t('Every Week on %1$s for the entire day until %2$s', [$days, $conclusion]),
+			[false, true, false] => $this->l10n->t('Every Week on %1$s between %2$s - %3$s', [$days, $startTime, $endTime]),
+			[false, true, true] => $this->l10n->t('Every Week on %1$s between %2$s - %3$s until %4$s', [$days, $startTime, $endTime, $conclusion]),
+			[true, false, false] => $this->l10n->t('Every %1$d Weeks on %2$s for the entire day', [$interval, $days]),
+			[true, false, true] => $this->l10n->t('Every %1$d Weeks on %2$s for the entire day until %3$s', [$interval, $days, $conclusion]),
+			[true, true, false] => $this->l10n->t('Every %1$d Weeks on %2$s between %3$s - %4$s', [$interval, $days, $startTime, $endTime]),
+			[true, true, true] => $this->l10n->t('Every %1$d Weeks on %2$s between %3$s - %4$s until %5$s', [$interval, $days, $startTime, $endTime, $conclusion]),
+			default => $this->l10n->t('Could not generate event recurrence statement')
+		};
 
-			//event that spans over multiple days
-			$localeStart = $this->l10n->l('date', $dtstartDt, ['width' => 'medium']);
-			$localeEnd = $this->l10n->l('date', $dtendDt, ['width' => 'medium']);
+	}
 
-			return $localeStart . ' - ' . $localeEnd;
-		}
+	/**
+	 * genarates a when string for a monthly precision/frequency
+	 *
+	 * @since 30.0.0
+	 *
+	 * @param EventReader $er
+	 *
+	 * @return string
+	 */
+	public function generateWhenStringRecurringMonthly(EventReader $er): string {
+		
+		// initialize
+		$interval = (int) $er->recurringInterval();
+		$startTime = '';
+		$endTime = '';
+		$conclusion = '';
+		// days of month
+		if ($er->recurringPattern() === 'R') {
+			$days = implode(', ', array_map(function ($value) { return $this->localizeRelativePositionName($value); }, $er->recurringRelativePositionNamed())) . ' ' .
+					implode(', ', array_map(function ($value) { return $this->localizeDayName($value); }, $er->recurringDaysOfWeekNamed()));
+		} else {
+			$days = implode(', ', $er->recurringDaysOfMonth());
+		}
+		// time of the day
+		if (!$er->entireDay()) {
+			$startTime = $this->l10n->l('time', $er->startDateTime(), ['width' => 'short']);
+			$startTime .= $er->startTimeZone() != $er->endTimeZone() ? ' (' . $er->startTimeZone()->getName() . ')' : '';
+			$endTime = $this->l10n->l('time', $er->endDateTime(), ['width' => 'short']) . ' (' . $er->endTimeZone()->getName() . ')';
+		}
+		// conclusion
+		if ($er->recurringConcludes()) {
+			$conclusion = $this->l10n->l('date', $er->recurringConcludesOn(), ['width' => 'long']);
+		}
+		// generate localized when string
+		return match ([($interval > 1), !empty($startTime), !empty($conclusion)]) {
+			[false, false, false] => $this->l10n->t('Every Month on the %1$s for the entire day', [$days]),
+			[false, false, true] => $this->l10n->t('Every Month on the %1$s for the entire day until %2$s', [$days, $conclusion]),
+			[false, true, false] => $this->l10n->t('Every Month on the %1$s between %2$s - %3$s', [$days, $startTime, $endTime]),
+			[false, true, true] => $this->l10n->t('Every Month on the %1$s between %2$s - %3$s until %4$s', [$days, $startTime, $endTime, $conclusion]),
+			[true, false, false] => $this->l10n->t('Every %1$d Months on the %2$s for the entire day', [$interval, $days]),
+			[true, false, true] => $this->l10n->t('Every %1$d Months on the %2$s for the entire day until %3$s', [$interval, $days, $conclusion]),
+			[true, true, false] => $this->l10n->t('Every %1$d Months on the %2$s between %3$s - %4$s', [$interval, $days, $startTime, $endTime]),
+			[true, true, true] => $this->l10n->t('Every %1$d Months on the %2$s between %3$s - %4$s until %5$s', [$interval, $days, $startTime, $endTime, $conclusion]),
+			default => $this->l10n->t('Could not generate event recurrence statement')
+		};
+	}
 
-		/** @var Property\ICalendar\DateTime $dtstart */
-		/** @var Property\ICalendar\DateTime $dtend */
-		$isFloating = $dtstart->isFloating();
-		$startTimezone = $endTimezone = null;
-		if (!$isFloating) {
-			$prop = $dtstart->offsetGet('TZID');
-			if ($prop instanceof Parameter) {
-				$startTimezone = $prop->getValue();
-			}
+	/**
+	 * genarates a when string for a yearly precision/frequency
+	 *
+	 * @since 30.0.0
+	 *
+	 * @param EventReader $er
+	 *
+	 * @return string
+	 */
+	public function generateWhenStringRecurringYearly(EventReader $er): string {
+		
+		// initialize
+		$interval = (int) $er->recurringInterval();
+		$startTime = '';
+		$endTime = '';
+		$conclusion = '';
+		// months of year
+		$months = implode(', ', array_map(function ($value) { return $this->localizeMonthName($value); }, $er->recurringMonthsOfYearNamed()));
+		// days of month
+		if ($er->recurringPattern() === 'R') {
+			$days = implode(', ', array_map(function ($value) { return $this->localizeRelativePositionName($value); }, $er->recurringRelativePositionNamed())) . ' ' .
+					implode(', ', array_map(function ($value) { return $this->localizeDayName($value); }, $er->recurringDaysOfWeekNamed()));
+		} else {
+			$days = $er->startDateTime()->format('jS');
+		}
+		// time of the day
+		if (!$er->entireDay()) {
+			$startTime = $this->l10n->l('time', $er->startDateTime(), ['width' => 'short']);
+			$startTime .= $er->startTimeZone() != $er->endTimeZone() ? ' (' . $er->startTimeZone()->getName() . ')' : '';
+			$endTime = $this->l10n->l('time', $er->endDateTime(), ['width' => 'short']) . ' (' . $er->endTimeZone()->getName() . ')';
+		}
+		// conclusion
+		if ($er->recurringConcludes()) {
+			$conclusion = $this->l10n->l('date', $er->recurringConcludesOn(), ['width' => 'long']);
+		}
+		// generate localized when string
+		return match ([($interval > 1), !empty($startTime), !empty($conclusion)]) {
+			[false, false, false] => $this->l10n->t('Every Year in %1$s on the %2$s for the entire day', [$months, $days]),
+			[false, false, true] => $this->l10n->t('Every Year in %1$s on the %2$s for the entire day until %3$s', [$months, $days, $conclusion]),
+			[false, true, false] => $this->l10n->t('Every Year in %1$s on the %2$s between %3$s - %4$s', [$months, $days, $startTime, $endTime]),
+			[false, true, true] => $this->l10n->t('Every Year in %1$s on the %2$s between %3$s - %4$s until %5$s', [$months, $days, $startTime, $endTime, $conclusion]),
+			[true, false, false] => $this->l10n->t('Every %1$d Years in %2$s on the %3$s for the entire day', [$interval, $months, $days]),
+			[true, false, true] => $this->l10n->t('Every %1$d Years in %2$s on the %3$s for the entire day until %4$s', [$interval, $months,  $days, $conclusion]),
+			[true, true, false] => $this->l10n->t('Every %1$d Years in %2$s on the %3$s between %4$s - %5$s', [$interval, $months, $days, $startTime, $endTime]),
+			[true, true, true] => $this->l10n->t('Every %1$d Years in %2$s on the %3$s between %4$s - %5$s until %6$s', [$interval, $months, $days, $startTime, $endTime, $conclusion]),
+			default => $this->l10n->t('Could not generate event recurrence statement')
+		};
+	}
 
-			$prop = $dtend->offsetGet('TZID');
-			if ($prop instanceof Parameter) {
-				$endTimezone = $prop->getValue();
+	/**
+	 * genarates a when string for a fixed precision/frequency
+	 *
+	 * @since 30.0.0
+	 *
+	 * @param EventReader $er
+	 *
+	 * @return string
+	 */
+	public function generateWhenStringRecurringFixed(EventReader $er): string {
+		// initialize
+		$startTime = '';
+		$endTime = '';
+		$conclusion = '';
+		// time of the day
+		if (!$er->entireDay()) {
+			$startTime = $this->l10n->l('time', $er->startDateTime(), ['width' => 'short']);
+			$startTime .= $er->startTimeZone() != $er->endTimeZone() ? ' (' . $er->startTimeZone()->getName() . ')' : '';
+			$endTime = $this->l10n->l('time', $er->endDateTime(), ['width' => 'short']) . ' (' . $er->endTimeZone()->getName() . ')';
+		}
+		// conclusion
+		$conclusion = $this->l10n->l('date', $er->recurringConcludesOn(), ['width' => 'long']);
+		// generate localized when string
+		return match (!empty($startTime)) {
+			false => $this->l10n->t('On specific dates for the entire day until %1$s', [$conclusion]),
+			true => $this->l10n->t('On specific dates between %1$s - %2$s until %3$s', [$startTime, $endTime, $conclusion]),
+		};
+	}
+	
+	/**
+	 * genarates a occurring next string for a recurring event
+	 *
+	 * @since 30.0.0
+	 *
+	 * @param EventReader $er
+	 *
+	 * @return string
+	 */
+	public function generateOccurringString(EventReader $er): string {
+
+		// reset to initial occurance
+		$er->recurrenceRewind();
+		// forward to current date
+		$er->recurrenceAdvanceTo($this->timeFactory->getDateTime());
+		// calculate time differnce from now to start of next event occurance and minimize it
+		$occuranceIn = $this->minimizeInterval($this->timeFactory->getDateTime()->diff($er->recurrenceDate()));
+		// store next occurance value
+		$occurance = $this->l10n->l('date', $er->recurrenceDate(), ['width' => 'long']);
+		// forward one occurance
+		$er->recurrenceAdvance();
+		// evaluate if occurance is valid
+		if ($er->recurrenceDate() !== null) {
+			// store following occurance value
+			$occurance2 = $this->l10n->l('date', $er->recurrenceDate(), ['width' => 'long']);
+			// forward one occurance
+			$er->recurrenceAdvance();
+			// evaluate if occurance is valid
+			if ($er->recurrenceDate()) {
+				// store following occurance value
+				$occurance3 = $this->l10n->l('date', $er->recurrenceDate(), ['width' => 'long']);
 			}
 		}
+		// generate occurance string
+		return match ([($occuranceIn[0] > 1), !empty($occurance2), !empty($occurance3)]) {
+			[false, false, false] => $this->l10n->t('In a %1$s on %2$s', [$occuranceIn[1], $occurance]),
+			[false, true, false] => $this->l10n->t('In a %1$s on %2$s then on %3$s', [$occuranceIn[1], $occurance, $occurance2]),
+			[false, true, true] => $this->l10n->t('In a %1$s on %2$s then on %3$s and %4$s', [$occuranceIn[1], $occurance, $occurance2, $occurance3]),
+			[true, false, false] => $this->l10n->t('In %1$s %2$s on %3$s', [$occuranceIn[0], $occuranceIn[1], $occurance]),
+			[true, true, false] => $this->l10n->t('In %1$s %2$s on %3$s then on %4$s', [$occuranceIn[0], $occuranceIn[1], $occurance, $occurance2]),
+			[true, true, true] => $this->l10n->t('In %1$s %2$s on %3$s then on %4$s and %5$s', [$occuranceIn[0], $occuranceIn[1], $occurance, $occurance2, $occurance3]),
+			default => $this->l10n->t('Could not generate next recurrence statement')
+		};
 
-		$localeStart = $this->l10n->l('weekdayName', $dtstartDt, ['width' => 'abbreviated']) . ', ' .
-			$this->l10n->l('datetime', $dtstartDt, ['width' => 'medium|short']);
-
-		// always show full date with timezone if timezones are different
-		if ($startTimezone !== $endTimezone) {
-			$localeEnd = $this->l10n->l('datetime', $dtendDt, ['width' => 'medium|short']);
-
-			return $localeStart . ' (' . $startTimezone . ') - ' .
-				$localeEnd . ' (' . $endTimezone . ')';
-		}
-
-		// show only end time if date is the same
-		if ($dtstartDt->format('Y-m-d') === $dtendDt->format('Y-m-d')) {
-			$localeEnd = $this->l10n->l('time', $dtendDt, ['width' => 'short']);
-		} else {
-			$localeEnd = $this->l10n->l('weekdayName', $dtendDt, ['width' => 'abbreviated']) . ', ' .
-				$this->l10n->l('datetime', $dtendDt, ['width' => 'medium|short']);
-		}
-
-		return $localeStart . ' - ' . $localeEnd . ' (' . $startTimezone . ')';
 	}
 
 	/**
@@ -261,12 +497,13 @@ class IMipService {
 	 * @return array
 	 */
 	public function buildCancelledBodyData(VEvent $vEvent): array {
+		// construct event reader
+		$eventReaderCurrent = new EventReader($vEvent);
 		$defaultVal = '';
 		$strikethrough = "<span style='text-decoration: line-through'>%s</span>";
 
-		$newMeetingWhen = $this->generateWhenString($vEvent);
+		$newMeetingWhen = $this->generateWhenString($eventReaderCurrent);
 		$newSummary = isset($vEvent->SUMMARY) && (string)$vEvent->SUMMARY !== '' ? (string)$vEvent->SUMMARY : $this->l10n->t('Untitled event');
-		;
 		$newDescription = isset($vEvent->DESCRIPTION) && (string)$vEvent->DESCRIPTION !== '' ? (string)$vEvent->DESCRIPTION : $defaultVal;
 		$newUrl = isset($vEvent->URL) && (string)$vEvent->URL !== '' ? sprintf('<a href="%1$s">%1$s</a>', $vEvent->URL) : $defaultVal;
 		$newLocation = isset($vEvent->LOCATION) && (string)$vEvent->LOCATION !== '' ? (string)$vEvent->LOCATION : $defaultVal;
@@ -522,7 +759,7 @@ class IMipService {
 			$data['meeting_title_html'] ?? $data['meeting_title'], $this->l10n->t('Title:'),
 			$this->getAbsoluteImagePath('caldav/title.png'), $data['meeting_title'], '', IMipPlugin::IMIP_INDENT);
 		if ($data['meeting_when'] !== '') {
-			$template->addBodyListItem($data['meeting_when_html'] ?? $data['meeting_when'], $this->l10n->t('Date and time:'),
+			$template->addBodyListItem($data['meeting_when_html'] ?? $data['meeting_when'], $this->l10n->t('When:'),
 				$this->getAbsoluteImagePath('caldav/time.png'), $data['meeting_when'], '', IMipPlugin::IMIP_INDENT);
 		}
 		if ($data['meeting_location'] !== '') {
@@ -533,6 +770,10 @@ class IMipService {
 			$template->addBodyListItem($data['meeting_url_html'] ?? $data['meeting_url'], $this->l10n->t('Link:'),
 				$this->getAbsoluteImagePath('caldav/link.png'), $data['meeting_url'], '', IMipPlugin::IMIP_INDENT);
 		}
+		if (isset($data['meeting_occurring'])) {
+			$template->addBodyListItem($data['meeting_occurring_html'] ?? $data['meeting_occurring'], $this->l10n->t('Occurring:'),
+				$this->getAbsoluteImagePath('caldav/time.png'), $data['meeting_occurring'], '', IMipPlugin::IMIP_INDENT);
+		}
 
 		$this->addAttendees($template, $vevent);
 
@@ -643,10 +884,104 @@ class IMipService {
 			return false;
 		}
 		$type = $cuType->getValue() ?? 'INDIVIDUAL';
-		if (\in_array(strtoupper($type), ['RESOURCE', 'ROOM'], true)) {
+		if (\in_array(strtoupper($type), ['RESOURCE', 'ROOM', 'UNKNOWN'], true)) {
 			// Don't send emails to things
 			return true;
 		}
 		return false;
 	}
+
+	public function minimizeInterval(\DateInterval $dateInterval): array {
+		// evaluate if time interval is in the past
+		if ($dateInterval->invert == 1) {
+			return [1, 'the past'];
+		}
+		// evaluate interval parts and return smallest time period
+		if ($dateInterval->y > 0) {
+			$interval = $dateInterval->y;
+			$scale = ($dateInterval->y > 1) ? 'years' : 'year';
+		} elseif ($dateInterval->m > 0) {
+			$interval = $dateInterval->m;
+			$scale = ($dateInterval->m > 1) ? 'months' : 'month';
+		} elseif ($dateInterval->d >= 7) {
+			$interval = (int)($dateInterval->d / 7);
+			$scale = ((int)($dateInterval->d / 7) > 1) ? 'weeks' : 'week';
+		} elseif ($dateInterval->d > 0) {
+			$interval = $dateInterval->d;
+			$scale = ($dateInterval->d > 1) ? 'days' : 'day';
+		} elseif ($dateInterval->h > 0) {
+			$interval = $dateInterval->h;
+			$scale = ($dateInterval->h > 1) ? 'hours' : 'hour';
+		} else {
+			$interval = $dateInterval->i;
+			$scale = 'minutes';
+		}
+
+		return [$interval, $scale];
+	}
+
+	/**
+	 * Localizes week day names to another language
+	 *
+	 * @param string $value
+	 *
+	 * @return string
+	 */
+	public function localizeDayName(string $value): string {
+		return match ($value) {
+			'Monday' => $this->l10n->t('Monday'),
+			'Tuesday' => $this->l10n->t('Tuesday'),
+			'Wednesday' => $this->l10n->t('Wednesday'),
+			'Thursday' => $this->l10n->t('Thursday'),
+			'Friday' => $this->l10n->t('Friday'),
+			'Saturday' => $this->l10n->t('Saturday'),
+			'Sunday' => $this->l10n->t('Sunday'),
+		};
+	}
+
+	/**
+	 * Localizes month names to another language
+	 *
+	 * @param string $value
+	 *
+	 * @return string
+	 */
+	public function localizeMonthName(string $value): string {
+		return match ($value) {
+			'January' => $this->l10n->t('January'),
+			'February' => $this->l10n->t('February'),
+			'March' => $this->l10n->t('March'),
+			'April' => $this->l10n->t('April'),
+			'May' => $this->l10n->t('May'),
+			'June' => $this->l10n->t('June'),
+			'July' => $this->l10n->t('July'),
+			'August' => $this->l10n->t('August'),
+			'September' => $this->l10n->t('September'),
+			'October' => $this->l10n->t('October'),
+			'November' => $this->l10n->t('November'),
+			'December' => $this->l10n->t('December'),
+		};
+	}
+
+	/**
+	 * Localizes relative position names to another language
+	 *
+	 * @param string $value
+	 *
+	 * @return string
+	 */
+	public function localizeRelativePositionName(string $value): string {
+		return match ($value) {
+			'First' => $this->l10n->t('First'),
+			'Second' => $this->l10n->t('Second'),
+			'Third' => $this->l10n->t('Third'),
+			'Fourth' => $this->l10n->t('Fourth'),
+			'Fifty' => $this->l10n->t('Fifty'),
+			'Last' => $this->l10n->t('Last'),
+			'Second Last' => $this->l10n->t('Second Last'),
+			'Third Last' => $this->l10n->t('Third Last'),
+			'Fourth Last' => $this->l10n->t('Fourth Last'),
+			'Fifty Last' => $this->l10n->t('Fifty Last'),
+		};
+	}
 }