Allow only medium date or "N/A" in a field

[newatit]

This may sound a bit basic, but I am new at Access and am having a hard time trying to allow only a medium date entry or an N/A entry into a field.
I have tried to find help in the help files but have not been too successful.
Is it a validation rule?

 

[DonQuichote]

If it is a date field, why do you want to restrict the format? You can give a format to indicate how the values must be shown, but it is very user-unfriendly to force some format to input. A date field itself (in the table) has no format, only a value.