Changing first day of week in NSDatePickerControlMBS

I have a NSDatePickerControlMBS and want to change the first day of the week. According to the documentation the NSDatePickerControl has a calendar, which has a property FirstWeekday. From the Open event:

[code]CalendarLowerRef = CalendarLower.View
'set some other properties
CalendarLowerRef.datePickerElements = NSDatePickerMBS.NSYearMonthDayDatePickerElementFlag

dim theCal as new NSCalendarMBS
theCal.firstWeekday = 0 ’ also tried value of 1
CalendarLowerRef.calendar = theCal[/code]

But the calendar doesn’t change. What am I doing wrong here?

macOS 10.11., Xojo 2018r1.1, 64bit, latest MBS

https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/DataFormatting/DataFormatting.html#//apple_ref/doc/uid/10000029i?language=objc

and you know that the original NSCalendar works by default with YOUR calendar (the currently running locale).

FirstWeekDay:
https://developer.apple.com/documentation/foundation/nscalendar/1408310-firstweekday

Thanks, Emile. I had forgotten about this (never noticed the setting before, actually).