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A Quick History Lesson

The Egyptians first came up with the idea of adding a leap day
once every four years to keep the calendar in sync with the solar year.
Later, the Romans adopted this solution for their calendar,
and they became the first to designate February 29 as the leap day.

But Wait! It’s Not Quite that Simple!

The math seems to work out beautifully
when you add an extra day to the calendar every four years
to compensate for the extra quarter of a day in the solar year.
However, the solar year is just about 365 ¼ days long—but not exactly!
The exact length of a solar year is actually
11 minutes and 14 seconds less than 365 ¼ days.
That means that even if you add a leap day every four years,
the calendar would still overshoot the solar year by a little bit—
11 minutes and 14 seconds per year.
These minutes and seconds really start to add up: after 128 years,
the calendar would gain an entire extra day. So, the leap year rule,
“add a leap year every four years” was a good rule, but not good enough!

Calendar Correction, Part II

To rectify the situation, the creators of our calendar
(the Gregorian calendar, introduced in 1582)
decided to omit leap years three times every four hundred years.
This would shorten the calendar every so often
and rid it of the annual excess of 11 minutes and 14 seconds.
So in addition to the rule that a leap year occurs every four years,
a new rule was added:
a century year is not a leap year unless it is evenly divisible by 400.
This rule manages to eliminate three leap years every few hundred years.

So now you know!