Mask edit with dates - strange behavior

I'm having a strange problem with MaskEdit with a date.  I'm
using the mask '!90/00/00 ;1; ', and I've tried variations.  The
problem is when editing a field that had a date in it, when the
month has two digits.  Ig I go to the end of the field, and
press Backspace, if the month has 1 digit, it clears the field
properly.  If it has 2 digits then (1) backspace clears only one
character at the time, and (2) it clears the slashes. leaving
digits.  For instance, if the field contains 12/23/99, pressing
backspace a few times will leave "2 3" or "2 3 9" in the field.
It leaves the digit before the slash (and maybe the last digit),
and elimates the slashes.  How can I fix this problem?

Jud McCranie