UTC is defined in terms of days. UTC calendar dates are presumed to use the Gregorian calendar. 2023-01-01 in the Gregorian calendar maps to a specific day (J

UTC is defined in terms of days. UTC calendar dates are presumed to use the Gregorian calendar. 2023-01-01 in the Gregorian calendar maps to a specific day (Julian day 2459946). If you wanted to use a different calendar, you would need to map it to Julian days also to convert between the two. The idea that the world would suddenly change to a new calendar is highly unlikely, and if it did happen it is not at all clear what the correct behavior would be for future dates that had been specified before the switch.