Why we have a leap year every four years — almost

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Another leap year is upon us — but why is it even here at all? As you might expect, there's some math involved.

We add a day to the calendar every time the year is divisible by four, such as 2016, 2020 or 2024, unless that year is divisible by 100. Exceptions are also made when the year is divisible by 400.

A year, or the time it takes the earth to make a revolution around the sun, is thought to be 365 days. It's actually 365.2422 days, which means that we cheat ourselves out of almost six hours each year.

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Without a leap day, the calendar would be off by about 24 days in 100 years. That means that spring, which usually falls on or around March 21, would start sometime in late February.

So, why not just add a day every four years and call it quits? Even doing that, we still end up cheating ourselves out of about 18 hours in 100 years.

Remember the rules about the years being divisible by 100 and 400? That's where those come into play.

If a leap year is supposed to be at the start of the century, like 2100, that leap year is skipped. Unless — you guessed it — that year is divisible by 400.

Following these rules, we can keep our calendar accurate to the day for a whopping 3,333 years.