Not all dates from some alternate calendar systems can be accommodated by QDate, for example, in Persian calendar, the second month has 31 days. QCalendar::YearMonthDay is a data structure to solve the problem.
alternateDateReady is not changed since more changes will depend on
some changes in KF6.
BUG: 463196 FIXED-IN: 5.26.5
(cherry picked from commit 64115353)