Andreas - I once worked on a PB v6.5 app (Classic/PFC) where users entered time-duration values (00:01:30 = 1 minute, 30 seconds, for example) and like you, the users wanted the input field to be empty if there was no value. For speed/productivity reasons they also wanted be able to enter values like "15", "1m", "90", or "1h" (for 15 seconds, 1 minute, 1 minute 30 seconds and 1 hour, respectively). Edit masks were far too inflexible, so we used a simple edit column object and interpreted/validated the input in the ItemChanged event and formatted the validated input value in a standard manner for presentation back to the user.
If I recall correctly, the actual time-duration data columns were defined as 32-bit unsigned longs in the database table, so we created a formatted string representation of each time duration (long) value in the DW's SELECT statement and that is what the user interaction affected, then we updated the actual long integer table column value via the validation code. Since we were using the PFC, we created a DW service object to handle the details, coded it once and used it in dozens of places throughout the app with minimal code.
We didn't have to resort to capturing keystrokes or try to coerce the edit mask style into doing things it wasn't capable of doing. Most importantly, our user community was very happy their request was heard and addressed. I don't see any reason a similar approach wouldn't work in your situation where you're working with time-of-day values, especially if you are using some kind of object framework in the app.
Food for thought.
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