Each rule holds across eras

All 12 scenarios hold

A leap year is a year with an extra day added to the calendar — February 29 — to keep the calendar year aligned with Earth’s orbit around the Sun. * A normal year has 365 days, but Earth takes about 365.2422 days to orbit the Sun. * To account for this extra fraction, we add one day every 4 years. * Exception: Years divisible by 100 are not leap years unless they are also divisible by 400.