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Created attachment 7627 [details]
Console app to demonstrate brokenness
(Seen against 3.6.0 on my Mac and 3.3.0 on my Windows box.)
I'm slightly surprised that Mono even claims to support the Um Al Qura calendar given that it's documented to require data licensed from the Saudi government, but it does. Unfortunately, it doesn't work.
The MS implementation only supports a very small range of years (183 at the moment) whereas Mono *claims* to support most of the range of DateTime (622-9999). However, it's not even internally consistent.
The attached test program shows the number of days in year 2 via a) calling GetDaysInYear; b) calling GetDaysInMonth for all the months and summing the results. The two values differ.
I don't mind the calendar not being supported - I'd just prefer it if it didn't *pretend* to supported.
I think Um Al Qura calendar does not necessary need Saudi government permission. It's only that Microsoft uses "old" table based (observed) data instead of Tabular Islamic calendar.
That still does not mean we are not buggy
Thank you for the detailed bug report.
The pull request for this issue can be found in the link below.
Fixed in master b946785022acf83db313daf08ae6b5f211dac0e5.