The real “official” date is December 14, when the Electoral College formally votes. Even Trump has pointed to this specific date as the date when he would “vacate the White House” if he loses the electoral vote. Even now there are still Republicans e.g. in PA still trying to overturn the election, even today. It is of course completely without any chance of succeeding though still theoretically possible…until December 14. Once the electors have voted, it is officially over. If Republicans controlled the next Congress, they would still have another shot there to dispute the electors or something, but obviously Nancy Pelosi isn’t going to tolerate that kind of nonsense in the House.