[bug-58468] implement DAYS function

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1901363 13f79535-47bb-0310-9956-ffa450edef68

poi/src/test/java/org/apache/poi/ss/formula/functions/TestDays.java

@@ -45,6 +45,7 @@ public class TestDays {
             HSSFCell cell = wb.getSheetAt(0).getRow(0).createCell(12);
             assertDouble(fe, cell, "DAYS(\"15-MAR-2021\",\"1-FEB-2021\")", 42, 0.00000000001);
             assertDouble(fe, cell, "DAYS(A2,A3)", 364, 0.00000000001);
+            assertDouble(fe, cell, "DAYS(\"1-FEB-2021\", \"15-MAR-2021\")", -42, 0.00000000001);
         }
     }