February 29 in the Gregorian calendar, the most widely used today, is a date that occurs only once every four years, in years evenly divisible by 4, such as 1976, 1996, 2000, 2012 or 2016 (with the exception of century years not divisible by 400, such as 1900). These are called leap years. February 29 is the 60th day of the Gregorian calendar in such a year, with 306 days remaining until the end of that year. It is also known as a leap day.
Although most years of the modern calendar have 365 days, a complete revolution around the sun takes approximately 365 days and 6 hours. Every four years, as an extra 24 hours have accumulated, one extra day is added to keep the count coordinated with the sun's apparent position.
It is however slightly inaccurate to calculate an additional 6 hours each year as the time actually taken for the earth to complete a revolution around the sun is 365 days, 5 hours and 49 minutes. To compensate for the 11-minute difference, a century year is not a leap year unless it is also evenly divisible by 400. This means that 1600 and 2000 were leap years, as will be 2400 and 2800, but 1800 and 1900 were not, and neither will 2100 and 2200 be.
The Gregorian calendar repeats itself every 400 years, which is exactly 20871 weeks including 97 leap days. Over this period February 29 falls 13 times on a Sunday, Tuesday or Thursday; 14 times on a Friday or Saturday; and 15 times on a Monday or Wednesday.
The concepts of the leap year and leap day are distinct from the leap second, which results from changes in the Earth's rotational speed.
The leap day was introduced as part of the Julian reform. The day following the Terminalia (February 23) was doubled, forming the "bis sextum"—literally 'double sixth', since February 24 was 'the sixth day before the Kalends of March' using Roman inclusive counting (March 1 was the 'first day'). Although exceptions exist, the first day of the bis sextum (February 24) was usually regarded as the intercalated or "bissextile" day since the third century. February 29 came to be regarded as the leap day when the Roman system of numbering days was replaced by sequential numbering in the late Middle Ages.
An English law of 1256 decreed that in leap years, the leap day and the day before (February 25 & 24) are to be reckoned as one day for the purpose of calculating when a full year had passed. In England and Wales a person born on February 29 legally reaches the age of 18 or 21 on February 28 of the relevant year. In the European Union, February 29 officially became the leap day only in 2000.
In cases of New Zealand citizens, the NZ Parliament has decreed that if a date of birth was February 29, in non-leap years the legal birth date date shall be the preceding day, the 28th. This is affirmed in § 2(2) of the Land Transport Act 1999.
Full article ▸