What is a leap day?
2100 will not be a leap year. In the calendar we use today, years ending in 00 need an extra check: can the year number be split into groups of 400 with nothing left over? For 2100, the answer is no.
But first, what is a leap year? An ordinary year has 365 days. A leap year has 366, with one extra date: 29 February.
We need that extra day because a full cycle of seasons takes a little longer than 365 days: almost another quarter of a day. Those small amounts of time add up.
Adding a day roughly every four years helps our calendar stay in step with the seasons. The special check for years ending in 00 makes that match more accurate.
Why “every four years” is not the whole story
The leftover time is a little less than a quarter of a day. Adding a whole day every four years therefore adds slightly too much.
The calendar fixes this by leaving out a few leap days. The extra rule applies to years ending in 00, such as 1900, 2000 and 2100.
These rules belong to the Gregorian calendar, the calendar widely used for everyday dates today.
Follow the checks one at a time
First, a small maths phrase: “divisible by four” means you can split a number into groups of four with nothing left over. For example, 12 makes three complete groups of four; 13 leaves one over.
- Can the year number be divided by 4 with nothing left over? If no, it is not a leap year. If yes, continue.
- Does the year end in 00? If no, it is a leap year. If yes, there is one last check.
- Can it be divided by 400 with nothing left over? If yes, keep the leap day. If no, skip it.
Compare 2000 and 2100
| Year | Groups of 400 | Leap day? |
|---|---|---|
| 2000 | Five complete groups; nothing left over | Yes |
| 2100 | Five complete groups, with 100 left over | No |
| 2400 | Six complete groups; nothing left over | Yes |
That is why 2100 will not have 29 February. It does pass the four-year check, but fails the extra check for years ending in 00.
An old date needs one extra question
Countries did not all start using this calendar at once. When reading about an old event, ask which calendar was in use there.
Next, see what happened when Britain changed its calendar in 1752. The date jumped, but people did not lose eleven days of their lives.