First is whether we are still using the same calendar base date. The currently accepted international system is based on Christendom, but there are other calendars out there with different dates. You could see a switch over if another group becomes more dominant. Or you could get another system implemented entirely; France tried to change its base year to the French Revolution.
Second is if Earth is the only human inhabited planet. We are already seeing that the Martian day throws a lot of coordination up in the air, and that is without having human bases there. It is possible that Mars develops its own calendar that better fits Martian time. At that point, the only link for calendars across humanity would be the Unix Epoch.