People 'mind-blown' after realising leap years aren't always every 4 years

Forget everything you thought you knew about leap years.

There are some basic rules about how we measure time that we all accept to be true. There are 24 hours in a day, seven days in a week, and 365 days in a year. Except for once every four years when there are 366 days in the year - which we call a . But what if we told you that's ?

It's commonly taught that the reason we have leap years is because technically the Earth takes just over 365 days to go around the sun. To keep things simple, we don't measure the extra bit every year, instead, we save it up for four years when we have one full day extra.

But one user named Luc has left people mind-blown as he pointed out that there's actually a little-known rule that only comes into effect every 100 years - and it changes everything we thought we knew about our calendar system.

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He explained in a video: "Leap years are not just every four years. I am 33-years-old and I am just learning this. It doesn't affect me because I won't be alive when it's relevant, but just in case you wanted to know, leap years are every four years except when the year is divisible by 100. So the year 2100 will not be a leap year."

But things get even more complicated than that. You may remember that the year 2000 was a leap year, which doesn't fit this bizarre rule - and that's because there's another caveat many of us had no idea existed. Luc added: "The exception to the divide by 100 rule is if it can also be divided by 400."

This might all sound completely made up, but the science behind it is backed by the in the US, who explain the process in detail on their website.

The reason we have these rules in place is because a day isn't actually bang on 24 hours - it's precisely 23.262222 hours. When we add a leap year every four years, we make the calendar longer by more than 44 minutes, so we claw those minutes back again so that our seasons don't become out of sync.

The experts claim: "The rule is that if the year is divisible by 100 and not divisible by 400, the leap year is skipped. The year 2000 was a leap year, for example, but the years 1700, 1800, and 1900 were not. The next time a leap year will be skipped is the year 2100."

Commenters on Luc's video were left baffled by the revelation. Many declared scientists were simply "making it up as they go", while others demanded to know "who came up with" the confusing system.

One person said: "This feels like when my kids try making up their own rules for board games." And another added: "I don't understand how leap years work and I think that I'm just not meant to."

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