Field Validation on a Date Field

I am trying to put a Field Validation on a Date field on a Custom Object.  The Date field is a custom field as well.  The logic says basically that only Mark can change this date, set this date or remove this date.  the logic below works for preventing a date from being changed, but not preventing someone from entering a date when it is blank to start or removing a date.  I've tried IS NULL and IS NOT NULL logic but doesn't seem to work on a date field.

(PRE('<dImplementation_Close_Date_ITAG>') = [<dImplementation_Close_Date_ITAG>]) OR (UserValue('Alias') = 'Mark Facer')