Use cmake TIMESTAMP function

because it is not only platform independent
but also allows to override the build date
This helps to make tigervnc builds reproducible

See https://reproducible-builds.org/ for why this is good

Also adds UTC flag, to be independent of timezone.
Also changes JAVA_DATE format to ISO-8601 date format.

Requires cmake-2.8.11+ from 2013

CMakeLists.txt

@@ -2,7 +2,7 @@
 # Setup
 #
 
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 2.8.11)
 if(POLICY CMP0022)
   cmake_policy(SET CMP0022 OLD)
 endif()
@@ -43,9 +43,7 @@ if(MSVC)
 endif()
 
 if(NOT BUILD_TIMESTAMP)
-  set(BUILD_TIMESTAMP "")
-  execute_process(COMMAND "date" "+%Y-%m-%d %H:%M" OUTPUT_VARIABLE BUILD_TIMESTAMP)
-  string(REGEX REPLACE "\n" "" BUILD_TIMESTAMP ${BUILD_TIMESTAMP})
+  STRING(TIMESTAMP BUILD_TIMESTAMP "%Y-%m-%d %H:%M" UTC)
 endif()
 
 # Default to optimised builds instead of debug ones. Our code has no bugs ;)

java/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 2.8.11)
 
 project(tigervnc-java Java)
 if(NOT VERSION)
@@ -25,13 +25,10 @@ set(JAVA_KEYPASS NOTFOUND CACHE STRING "Password used to protect the private key
 set(JAVA_TSA_URL NOTFOUND CACHE STRING "URL of Time Stamping Authority (TSA)")
 
 if(NOT BUILD)
-	execute_process(COMMAND "date" "+%Y%m%d" OUTPUT_VARIABLE BUILD)
+	STRING(TIMESTAMP BUILD "%Y%m%d" UTC)
 endif()
-execute_process(COMMAND "date" "+%b %d %Y" OUTPUT_VARIABLE JAVA_DATE)
-execute_process(COMMAND "date" "+%H:%M:%S" OUTPUT_VARIABLE JAVA_TIME)
-string(REGEX REPLACE "\n" "" JAVA_DATE ${JAVA_DATE})
-string(REGEX REPLACE "\n" "" JAVA_TIME ${JAVA_TIME})
-string(REGEX REPLACE "\n" "" BUILD ${BUILD})
+STRING(TIMESTAMP JAVA_DATE "%Y-%m-%d" UTC)
+STRING(TIMESTAMP JAVA_TIME "%H:%M:%S" UTC)
 
 set(JAVA_SOURCES "")
 set(JAVA_CLASSES "")