When a “fixed” formatting string proves anything but fixed, then breaks without warning or even mention in a release note.
NSDatePicker
How Apple made changes to Foundation routines which format dates and times which can break their formatting when in 12-hour clock mode.
Why couldn’t a user specify times before 0100 and after 1300, when the DatePicker worked fine using the 24-hour clock?
The Eclectic Light Company 