The maths
We add an extra day to the year (nearly) every four years. Why? The hard maths part is that the time it takes for the Earth to travel around the Sun is not exactly 365 days. It takes about six hours longer, so after four years it’s taken approximately 24 hours longer to go around the sun.
The solar year is actually 365.242 days, but no calendar can work around that. So we need an occasional corrective mechanism – an extra day every fourth year – which exist to keep our calendar synchronised with the solar year.
It allows the seasons to stay consistently aligned to the same months each year.
What would happen without them?
Over the centuries our seasons would slip out of sync, with the sun and moon no longer aligning with calendar months. Major events would fall in different seasons, like Christmas in summer. Would midsummer work for the fireworks of November 5th? Would Halloween be Halloween in the summer without pumpkins? No baby rabbits, eggs and daffodils for an Easter in the winter.
In the northern hemisphere we could have spring in September, summer in December, autumn in March and winter in July. Equally, in the southern hemisphere, the seasons would also eventually reverse.
It would affect when farmers plant their crops, seasonal activities, and cultural events linked to specific times of the year.
This graphic illustrates how the months would drift.
When are leap years?
Leap years must be divisible by four – except for century years, which must also be divisible by 400. This means that 2000 was a leap year, but 1700, 1800, and 1900 were not. The reason is again mathematical and is explained here.
What happens in leap years?
U.S. presidential elections happen to fall in leap years, as do the summer Olympics. One exception was the 2020 Japan games, delayed till 2021 due to the Covid pandemic. Another was when wars prevented them in 1916, 1940 and 1944. The 2024 Games will be in Paris.
Bachelors’ Day
Another quadrennial event, which is just tradition, is that women can propose marriage to men, on 29 February only. In days past, if a man was taking too long to pop the question, the woman was permitted to do so, but only once every four years. In some countries a man was penalised for refusing, by having to recompense the woman. Typically, a man who said ‘no’ was expected to buy the woman a silk gown or, more recently, a fur coat.
Getting married on 29 February, however, means you don’t have an annual anniversary. This could appeal to people who don’t want to bother celebrating annually, preferring to do something really special every four years.
Leaplings
About 5 million of the 8 billion global population were born on a 29 February. Having this birthday may be a conversation starter, but those leapling children are probably aggrieved that their true birthday only falls quadrennially.
A baby born on 29 February 2000 won’t have their 24th birthday this year, only their 6th. In non-leap years, some leaplings celebrate on 28 February, others on 1 March.
In UK law, however, leaplings become a year older not on 28 February, but 1 March.
Do you get paid for working?
If you are paid by the hour or by the day, you are paid for working on 29 February. However, for those on an annual salary, you have to work an extra day for free. You are paid a set amount for a year, so will earn 1/12 of your annual salary in February however many days it has. Perhaps an unpaid extra day of work for some. Perhaps leap days should be a bonus bank holiday?
Calendar hack
A centuries-old attempt to order the universe for human convenience, leap years are observed internationally and are here to stay. Not having them would be a problem eventually, but only for the generations of the far future.
