[The formula for determining when to add an extra day to the year is a bit complicated. It
goes like this:

*average* duration of our present day calendar is 365 + 1/4 - 1/100 + 1/400 = 365.2425 days,
which is pretty close to the actual length of the solar year of 365.2422 days. Our present
day yearly calendar is only three tenthousandths (0.0003) of a day too long.]

- Add an extra day every four years.
- Every one hundred years we skip a leap year.
- Every four hundred years we DO have the leap year that we would normally skip.

- Years evenly divisible by 4 are leap years.
- Except years evenly divisible by 100 are NOT leap years.
- EXCEPT years evenly divisible by 400 which ARE leap years.

It's interesting to note that there is no year A.D. 0 or 0 B.C.. The years jump from 1 B.C. to A.D. 1.

- Frequently Asked Questions about Calendars by Claus Tøndering

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