Clear Time Column

PowerBuilder 12.5

I have a datetime column which represents time with eidtMask hh:mm and format hh:mm.  If I write something to this field then I can't clear it so as to become null.  It will show 00:00 and treat it midnight.  In some cases I have written code to handle this situation (user selects all and presses backspace), but I would like to know if there is an easy way of doing this.