Happy birthday to everyone born on February 29. It's been four years, but today is the day all you leaplings can officially celebrate on your birthday.

For the rest of us it's an extra day of summer (or winter depending upon which hemisphere you're reading this in).

But did you know, you can't count on getting a bonus day every four years.

Why do we need a leap year?

Leap years help keep our calendars in synch with the seasons, said Jonti Horner, an astronomer at the University of Southern Queensland

"We have this idea that ... the year is exactly 365 days," Professor Horner said.

"Unfortunately, the universe doesn't actually work like that.

In fact, there are two kinds of years, both of which take a little more than 365 days to complete.

The first, known as a sidereal year, is the time it takes for the Earth to go around the Sun and return to the same place in the sky. It takes 365.256 days to complete.

But our calendar year is based on the time it takes to get from one vernal equinox to the next.

Known as the 'tropical year' it takes 365.24219 days.

It's a bit shorter than the sidereal year because the Earth wobbles on its axis.

"The seasons are due to which direction the Earth is pointing to in space, so in order to make the seasons align ...you've got a slightly different distance to cover," Professor Horner explained.

If we stuck with the notion a tropical year was only 365 days long, after 100 years our calendars would have shifted forward by 25 days.

Instead of the vernal equinox being on 21 March, it would be around 15 April.

"And that would confuse people because suddenly the seasons are out of whack with your calendar," Professor Horner said.

"So to remedy that we have an extra day almost every four years."

Why don't we have a leap year every four years?

Without leap years our calendar would be out of whack with our seasons. ( Wikimedia Commons: Jay8085 )

One of the earliest calendars to deal with the differences between time and the seasons was developed under the rule of Julius Caesar in 68 BC.

The Julian calendar added one day every four years to realign with the seasons.

At 365.25 days it was between a sidereal and tropical year.

"They got that wrong a couple of times at first, but they got it all ironed out by about 8 AD," Professor Horner said.

But the Julian calendar was out by a fortnight by the mid-1500s.

"If you add one day every four years, you're adding slightly more time than you need."

So in 1582 Pope Gregory XIII shifted the dates back by 10 days and came up with a new calendar that all countries use today.

Instead of having 100 leap years every 400 years the Gregorian calendar has 97.

This refines the number of days in a year down to 365.2425 days.

"Which is almost, but not exactly the same as 365.24219, but it's close enough that for hundreds, if not thousands, of years it will be bang on before starting to drift."

How is a leap year calculated?

Under the Gregorian calendar a leap year must be able to be divided by 4.

But if the year can be divided by 4 and by 100, it must also be divisible by 400 to count as a leap year.

So, for example, the year 2000 was a leap year, but 1900 was not. Although it can be divided by 4 and 100 (475 and 19) it can't be evenly divided by 400.

That means leap years will continue to happen like clockwork every four years up until 2096.

But 2100 will not be a leap year (apologies in advance to any babies born today but you're going to have to rethink your 80th birthday — or is it your 20th?).

Leap years will not happen on 2200 or 2300 either.

The eastern orthodox church uses another calendar known as the revised Julian calendar.

The maths of this calendar is even more complicated than the Gregorian calendar.

"The revised Julian calendar says when years are divisible by 100 they're not leap years unless years have remainders of 200 or 600 when you divide by 900," Professor Horner said.

But this calculation is even more precise — a year is 365.24222222 days — than the Gregorian calendar.

Luckily for everyone, leap years in the two calendars match up until 28 February 2800.