I’m using Xojo 2015.3, WebEdition.
I open in the example folder:
Web
Controls
CustomControls
jQuery Calendar.xojo_binary_project
I press run to compile the app. The web browser [OS X Safari 9.x] opens.
I see November and December 2015.
When I click at 10th of December, the Label says: Montag, 12. Oktober 2015
Then no click works.
That example seems to be working fine for me here with Safari 9 and 2015r3.
Perhaps I have the problem, because I have a german Mac.
In germany the first day of the week is a Monday. The example use Sunday.
Hmm. I try it again.
Ok, I think I found the problem:
In Germany the time format is:
TT.MM.JJJJ
In the USA: MM.TT.JJJJ
When I click on the 12th December 2015, the Label shows 10. Decemeber 2015.
In Numbers: 10.12.2015
So Xojo, jQuery or my Mac - have problems with months and days.
Because 12.10.2015 would be correct.
I have to do more trys. Is there a german developer, for example Ulrich from Xojo Germany, how could test it, too?
In the jQueryCalendar control, change the CurrentDateFormat property to match your localization.
Sounds not good:
Because when I change it to German Localization, it will work in Germany.
But then my users in USA, HongKong, GB, Spain will get problems. Difficult. 
Check out the Xojo.Locale class. You need to do the localization yourself.
[quote=226806:@Thomas Mueller]Sounds not good:
Because when I change it to German Localization, it will work in Germany.
But then my users in USA, HongKong, GB, Spain will get problems. Difficult. :-([/quote]
Put the localizations into a dynamic constant. It’ll deliver the correct thing automatically based on the selected language OR mofify the control and create the date object manually instead of using ParseDate.