Normally our calendar years are made up of 365 days, but every four years we add a day to make what is known as a ‘leap year’.
The Gregorian calendar, the most widely used across the globe, measures a year to be 365 days following the Earth's orbit around the Sun.
However, the Earth's orbit takes 365.24 days to complete its solar orbit and an extra day every four years is added to keep calendar seasons synchronised with solar seasons.
If we didn’t do this the seasons would shift around the calendar, and in 750 years’ time June would find itself in the middle of winter.
The modern calendar
The origins of the modern calendar as we know it date back to the Roman times.
Julius Caesar introduced the Julian calendar in 46 BC, which consists of 365 days with 366 in every fourth year.
However, in his calendar February had 30 days and August 29.
When Caesar Augustus became emperor he changed this so "his" month had the same amount as Caesar’s. He reduced February to 28 days to accommodate the amendment.
In 1582 the UK switched to the Gregorian calendar and worked out the Julian Calendar didn’t actually reflect the actual time the Earth takes to orbit the sun, or tropical year.
The Gregorian calendar follows three rules to determine which years are leap years: if the year can be divided by four it is a leap year, but if it can be evenly divided by 100 or by 400 then it is not.
Despite this complicated process the calendar is still not perfect, every 3,236 years it will be out by a day.
Comparably, the Julian calendar is currently lagging behind the Gregorian calendar by 13 days.
Leap day birthdays
Statistics expert Courtney Taylor, writing on website About.com, says around 0.07 per cent of the world’s population was born on a leap day, or 4.8 million people
1/30 Earth Day
Google celebrates Earth Day 2016
2/30 Ravi Shankar
Google marks Pandit Ravi Shankar's 96th birthday
3/30 Olympic Games in 1896
Google are celebrates the 120th anniversary of the modern Olympic Games in 1896
4/30 World Twenty20 final
Google celebrates the 2016 World Twenty20 cricket final between the West Indies and England with a doodle
5/30 William Morris
Google celebrates William Morris' 182 birthday with a doodle showcasing his most famous designs
6/30 St Patrick's Day
17 March 2016: St Patrick's Day
7/30 Caroline Herschel's 266th birthday
Caroline Herschel discovered eight comets and comprehensively catalogued the stars
8/30 Clara Rockmore's 105th birthday
Clara Rockmore was a pioneer of the Theremin musical instrument
9/30 International Women's Day
#OneDayIWill video marks International Woman's Day on 8 March, 2016
10/30 St David's Day 2016
Google marks St David's Day 2016 with a doodle
11/30 Leap Year
Google celebrates Leap Day 2016, February 28
12/30 Lantern Festival 2016
Google celebrates the last day of the Chinese New Year celebrations with a doodle of the Lantern Festival
13/30 Stethoscope Inventor, René Laennec
Google celebrate's René Laennec's 235th birthday
14/30 Valentine's Day 2016
Google celebrates Valentine's Day with a romantic Doodle
15/30 Dmitri Mendeleev
Google celebrate Dmitri Mendeleev's 182nd birthday
16/30 "The televisor" demonstartion
Google Doodle celebrates 90 years since the first demonstration of television or "the televisor" to the public
17/30 Professor Scoville
Google marks Professor Scoville’s 151st birthday
18/30 Sophie Taeuber-Arp
Google marks Sophie Taeuber-Arp's 127th birthday
19/30 Charles Perrault
Google celebrates author Charles Perrault's 388th birthday
20/30 Mountain of Butterflies discovery
Google celebrates the 41st anniversary of the discovery of the Mountain of Butterflies
21/30 Winter Solstice
Google celebrate the Winter Solstice 2015
22/30 St Andrew's Day 2015
Google marks St Andrew's Day 2015 with doodle featuring Scotland's flag and Loch Ness monster
23/30 41st anniversary of the discovery of 'Lucy'
Google marks the 41st anniversary of the discovery of 'Lucy', the name given to a collection of fossilised bones that once made up the skeleton of a hominid from the Australopithecus afarensis species, who lived in Ethiopia 3.2 million years ago
24/30 George Boole's 200th birthday
The Google Doodle on 2nd November to mark George Boole's contribution to science
25/30 Halloween 2015
Google celebrates Halloween 2015 using an interactive doodle game "Global Candy Cup"
26/30 Prague Astronomical Clock
Google celebrates the 605th anniversary of the Prague Astronomical Clock, one of the oldest functioning timepieces in the world
27/30 Autumnal Equinox
Google marks the autumnal equinox on 23 September 2015
28/30 The first electric traffic stop
Google marks the 101st anniversary of the first traffic signal system
29/30 New Horizons Pluto flyby
Google marks the Nasa probe's fly-past of Pluto on 14 July 2015
30/30 Father's Day 2015
Google marks Father's Day with an animated Doodle
Those born on a leap year are invited to join the Honour Society for Leap Year Babies, which currently has 11,000 members.
There are surprisingly few Leap Year traditions, but the custom of women proposing to men is the most famous.
The legend goes that in 5th Century Ireland, St Bridget complained to St Patrick about women having to wait too long for men to propose, to which St Patrick allowed women to propose on 29 February.
Although the more observant may note this could still result in a wait of nearly four years, the tradition has nonetheless gained popularity since the 19th Century.