Actually I only used Cat A in the example so people wouldnt bring RIE (extensions) etc into the picture
And youre mistaken about the Cat A being restricted to Flight Days. While the Cat A doesnt have a defined RI unlike the other categories, it can be Flight Days, Calender Days, Flight Cycles or even Flight Hours, The major distinction (other than the fact its variable) is that it cannot be extended (RIEs donot apply to Cat A MELs) - neither JAA nor FAA allows this
Independently though, my point was specific to the final moments before expiration (considering the count - be it flight day or calender day has already begun & is about to expire)