I can't think of a good way to impose a limit on just a particular section of the UltraMaskedEdit, though I'd think that it would be possible since the UltraDateTimeEditor has to do things like this to limit the number of days in a month.  You may have to derive the control (and possibly the owner) in order to control this behavior; another alternative would be to handle the Validating or BeforeExitEditMode events to manipulate the value or prevent the user from leaving the control.  At the very least, the latter would be much simpler to implement.

-Matt

 I can do nnn:nn, but the only problem I see is that someone could enter in 55:99 -- since there's not 99 minutes in an hour I'll have to correct that...