"Invalid Char in Field" error

I'm getting the same message every time I try to add or edit a date
        EDatabase Error:  Invalid Char in Field:  DOB   (for example).

I've tried using/not using edit masks, changing Regional settings in
Win95, etc., etc.   I can't imagine what is going on. Existing data is
fine, as long as I don't try to edit it  and post it or add a new

I would appreciate any thought anyone might have on this.
I'm using Delphi 3, Paradox 4.0 tables.