360Why do we have leap years?
A year on Earth, the time it takes to orbit the Sun, is about 365 and a quarter days. To keep the calendar in step with the seasons, an extra day, 29 February, is added almost every four years. Century years are leap years only if they divide by 400, so 2000 was one, but 2100 will not be.
The Earth takes about 365.2422 days to go around the Sun and return to the same point in its seasons. A calendar of exactly 365 days would slowly drift: after a century, the seasons would arrive almost a month late.
In 46 BC, Julius Caesar introduced the Julian calendar, adding one extra day every four years. That made the average year 365.25 days, slightly too long, so the calendar still drifted by about a day every 128 years.
By the 16th century, it was about ten days out. In 1582, the Gregorian calendar fixed this: century years would be leap years only if divisible by 400. So 1700, 1800 and 1900 were not leap years, 2000 was, and 2100 will not be.
Countries adopted the new calendar at different times. Britain and its colonies switched in 1752, skipping 11 days.
The Gregorian calendar is accurate to about one day in 3,000 years. People born on 29 February, sometimes called 'leaplings', usually celebrate on 28 February or 1 March in other years.
Sources: Royal Museums Greenwich · Time and Date · Britannica.