Under the Julian calendar, each 400 years had 300 regular years (109500 days) and 100 leap years (36600 days), for a total of 146100 days, averaging 365.25 days per year.

Under the Gregorian calendar, each 400 years had 303 regular years (110595 days) and 97 leap years (35502 days), for a total of 146097 days, averaging 365.2425 days per year.

In the paragraphs below, I will refer to this duration (365.2425 days) as a “standard” year. One thousand such years would be 365242.5 days.

**The 1000 years from Julian 1/1/1 to Julian 1/1/1000 contained** 750 regular years (273750 days) and 250 leap years (91500 days), totalling 365250 days, which is 7.5 days **more than a thousand standard years.**

A few centuries later, that 7.5 day error had grown to eleven days, which Pope Gregory deleted from the calendar. Therefore, although Julian 1/1/1001 to Julian 1/1/2001 would have also been 365250 days, the convention is to count these thousand years as running from **Julian 1/1/1001 to ***Gregorian* 1/1/2001 (or a year earlier than each).

That time comes to eleven days less than 365250 days, and deduct three more days for the lack of leap years in 1700, 1800, and 1900. This comes to 365236 days, which **is a full two weeks shorter than a thousand standard years.**

But in the long run, Gregory’s “Century Leap Year Rule” does average out in the long run. Add the two millennia together, and there have been 730486 days from **Julian 1/1/1 to Gregorian 1/1/2001, which is only one day longer than 2000 standard years,** and that one day will be compensated for by the lack of a leap day in 2100 and 2200.