I've created several date/time fields and added the folowing patterns using the patterns button.
For viewing
KK:MM
and for validation and edit
KK:MM|KK.MM
But i've got an error with these patterns (translated from Dutch, hope it make sense):
"The specified pattern can represent a date value"
and
"Pattern Processing: KK: MM | KK.MM is not compatible with the data format of the object. Define a compatible editing pattern."