### In summary

Every year between 1901 and 2099 divisible by four (including the year 2000) is a leap year, its month of February gaining an extra day: 29 February. This means the year 2012 was a leap year, the next one will be 2016, the next after 2020, etc.

### Background

We refer to the total time the earth needs to circle around the sun once (and go through all four seasons once) as a 'year'. We refer to the total time the earth needs to spin around its own axis (and go through its day-night cycle once) as a 'day'.

Precise astronomical measurements have confirmed that a year (presently and on average) contains 365 days, 5 hours, 48 minutes and 45.19... seconds, or 365.24219... days. It is, of course, far from practical to have a year contain a fractional number of days, and that is why the 'leap year', containing one extra day, has long been in use. The name 'leap year' comes from the fact that while a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, in a leap year the day of the week will advance two days (from March onwards) due to the year's extra day inserted at the end of February (thus 'leaping over' one of the days in the week).

In 46 BC, Julius Caesar introduced the so-called 'Julian' calendar, in which every fourth year was a leap year. On average, this produces a year of 365.25 days. However, this results in years being too long by approximately 365.25 - 365.24219... = 0.00781... days (or about 11 minutes and 15 seconds). By around 730 AD, science had advanced to the point that this error was recognized, but it took centuries before it was acted on.

In 1582, Pope Gregory XIII, to correct the drift of ten days accumulated over time in one go, ordered that Thursday, 4 October 1582 would be followed directly by Friday, 15 October 1582. He also ordered that from that time on, every period of 400 years would only contain 97 leap years. By this order, a year is approximately 365 + 97/400 = 365.2425 days in length, an approximation of the factual state of affairs which is sufficiently accurate for the time being. Naturally, Pope Gregory chose to introduce the correction at such a time that no Christian holidays would be left unobserved for the year. Yet still his order was widely resisted: many people believed that by leaping directly from 5 to 14 October 1582, they would literally lose ten days of their lifespan.

This 'Gregorian' calendar was immediately accepted in Catholic countries like Italy, Spain, Portugal and Poland. France, Luxembourg, Belgium and the then southern provinces of the Netherlands would follow soon after. The northern Netherlands only abandoned the Julian calendar at around 1700 AD, Great Britain waited until 1752, while the Soviet Union only made the switch in 1918. This has lead to some dating issues. The major British mathematician and physicist Sir Isaac Newton, for instance, was born on Christmas Day in 1642 according to the calendar used in England at the time - but for much of the world, it was already 4 January 1643. And the Russian October Revolution (of 25 October 1917) is commemorated on 7 November - the Julian calendar in 1918 being twelve days behind the Gregorian one, as compared to the ten days in 1582. *)

The Gregorian calendar spreads the 97 leap years over each 400-year period as follows:

- the base year has 365 days (resulting in 0 leap years per 400 years);
- exception to this rule: every year divisible by four becomes a leap year (resulting in 100 leap years per 400 years);
- exception to this rule: the last year of a century does not become a leap year (resulting in 96 leap years per 400 years);
- exception to this rule: if the last year of a century is divisible by 400, it becomes a leap year after all (resulting in the desired 97 leap years per 400 years).

The additional leap day was attached to February, as that month was the final month of the year until 456 AD, and therefore had the least amount of days. This latter fact is partially the result of the Roman Emperor Augustus shortening the month of February by a day so he could add this day to the month of August, named after him, and have it be equally long as the preceding month of July, named after Julius Caesar. So up until the year 456, the year started with the month of March (to have the year start in spring), which fact also explains why September, October, November and December literally mean the seventh, eighth, ninth and tenth month, respectively.

### Conclusion

The years 1700, 1800 and 1900 were no leap years, 2000 was, 2100, 2200 and 2300 will not be, etc. Everyone born after 1900 and deceased prior to 2100 will never witness a situation in which not every fourth year is a leap year.

**) Standardizing local time generally took even longer than standardizing dates. Different localities in the Netherlands kept their own clock right up until 1 May 1909. So the last day which had various towns in the Netherlands keeping different clocks was Friday 30 April 1909, which also just happened to be the day Queen Juliana was born.*

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