Why are you trying to enter a date in a field that should automatically enter today's date each time the record is created?
In any case, if you open Manage | Database | Fields and double click the field definition for this field, you'll open up field options. Click the validation tab and clear the check box labeled "existing values" and you'll no longer get this validation error message.
Ah. Thanks for two excellent tips. New to all of this. Didn't realize you could auto-enter. Now I see there's a tab for that. Also, found that hidden checkbox. Perfect!