What's in a leap year? Eternal youth, wedding bells and tech bugs

An extra day every four years, what's not to love?

The calendar quirk of February 29 keeps us in sync with the seasons but it has also spawned a host of rituals and superstitions, not to mention computer glitches, which AFP unpacks here:

Leap years have been with us since the 16th century, an invention of the Gregorian calendar, introduced to deal with a troublesome fraction in the solar year.

Bearing in mind it takes around 365.2422 days each year for the Earth to revolve around the sun, the extra snippet (around six hours a year) adds up over time.

Leap days regulate things -- without them we would fall out of sync with the seasons, causing havoc for farmers and their crops as well as school holidays.

Most leap years fall every four years, but as the extra snippet is not exactly six hours, they exclude years exactly divisible by 100.

However, years such as 1600, 2000 or 2400 are leap years as they are exactly divisible by 400.

For leap day babies, or leaplings, being born on 29 February may mean four times fewer birthdays, but it is also, as some like to claim, the key to eternal youth.

At least, that's what much-loved French screen star Michele Morgan liked to say during her lifetime, which lasted till the ripe old age of 96.

Among other famed or notorious leaplings are Spanish premier Pedro Sanchez, US rap star Ja Rule and serial killer Aileen Wuornos, incarnated by Charlize Theron in her Oscar-winning performance for "Monster".

With the chance of babies being born on a leap day at one in around 1,500, there are an estimated five million leaplings in the world today.

In Ireland, February 29 is known as Bachelor's Day or Ladies Privilege, when, tradition has it, women can propose to men rather than waiting to be wooed.

While some claim only a "Yes" answer is allowed, others say the man can decline, but must buy his admirer a gift.

The tradition received the Hollywood treatment in 2010 with "Leap Day" starring Amy Adams who follows her beau to Dublin in a bid to ensnare him into marriage by popping the question on the day.