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theBeardsley t1_iydrsga wrote

The Earth doesn't take exactly 365 days to orbit the sun, it's a bit closer to 365.25 days. So after four years, we have basically banked an extra day, and a leap year includes that extra day in the calendar to keep our calendar on track.

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Persist_and_Resist t1_iydsnnc wrote

The exceptions are years multiple by 400. Those we do not make leap years even though they are divisible by 4. And we do that because it is actually closer to 365.246 days to a year, and that makes it pretty much consistent over the course of even thousands of years.

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MagicalPizza21 t1_iydtgy8 wrote

The opposite - years divisible by 400 are leap years, but other years divisible by 100 but not 400 are not leap years. 2000 was a leap year and it's divisible by 400.

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