Leap years don't happen often so it's easy to forget when it's the next one.
They're needed to align our calendar with the sun, but it's very unfortunate if your birthday lands on one.
And for traditionalists, they pose the chance for a woman to ask a man to marry her, thanks to an old-age Irish belief called Bachelor's Day.
But why does the calendar have to change and when is the next one? Here's what you need to know:
Why does it happen?
Each year, our Gregorian calendar is made up of 365 days, except a leap year has 366 days.
An extra day is added because the amount of time it takes for the Earth to circle the sun is actually 356.24219 days - equating to 365 days, 5 hours, 48 minutes, and 45 seconds.
Time lost roughly adds up to 24 hours every four years, which is why a day is added at this interval.
This may not seem like much extra time but after 100 days, without the added day, we would have lost around 24 days.
They occur in February because it's the shortest month, and fall on the 29th - which doesn't exist in a common year.
Although it may seem like a simple process today, it's taken years of work since the Julian Calendar to try to get it right.
When is the next one?
The last leap year was in 2016, therefore the next one will land on Saturday, February 29, 2020.
'Leap year babies' are already talking about the prospect of celebrating their birthday on Twitter, with one user posting: "Next year is a leap year and I’m so excited, finally I get to have a real birthday again."
Even if it's not your birthday, make sure you make the most of it.
