This is a bug I run into each time in a few applications within ServiceNow
I've tested the things told above, here's the ennoying part:

IF you switch the month with the day it gets wonky.
dd-MM or dd/MM
If the date is set to 11 december it will be set to 12-11 not 11-12, if you go above the 12th of that month it gets invalid. Probably has to do with the UTC time etc.

I've reported this bug, as said by Kosh this should work for all date formats.