Once a date field is flagged as 'invalid' (e.g. the background goes to "red"), an "empty" value does not remove that 'invalid' flag (i.e.: red background).
Furthermore, one can't "save" until that 'invalid' field becomes valid by putting in a valid date (and then removing it, if one really wants to leave it blank).
Should only have to remove the 'invalid' value to "clear" the flagging.