The extra day hiding in your life
Every few years the calendar slips in a 29 February to keep itself in step with the Sun. Most of us barely notice, yet over a lifetime these bonus days add up to two or three weeks. They are also why your age in days is never just your age multiplied by 365. The beads above show each year of your life: the gold ones contained a 29 February, the plain ones did not, and the highlighted bead is the year you are in now.
Slide the age control to look ahead. The beads stretch into the future and you can see the gap in 2100, the first century year since 1900 that will skip its leap day. Anyone alive then will have a seven-year stretch without one, from 2096 to 2104.
How we calculate it
We check every year from your birth year onward and count the ones that have a 29 February falling on or after your birth date and on or before today. A year is a leap year if it divides by four, except century years, which must also divide by 400. That rule, introduced with the Gregorian calendar in 1582, makes the average calendar year 365.2425 days long, within about 27 seconds of the real solar year.
| Year | Leap year? | Why |
|---|---|---|
| 1900 | No | Century year not divisible by 400 |
| 2000 | Yes | Century year divisible by 400 |
| 2024 | Yes | Divisible by 4 |
| 2028 | Yes | Divisible by 4 |
| 2100 | No | Century year not divisible by 400 |
Worked example. Someone born on 12 April 1990 just missed the leap day of 1988, but 29 February 1992 came after their birth, so it counts. Their leap days are 1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020 and 2024, which makes nine by 2026. The next is 29 February 2028.
For leap-day babies
If you were born on 29 February, you are one of roughly five million “leaplings” worldwide. Your birthday only appears on the calendar every four years, so you can say you have had a quarter as many birthdays as your age suggests. In common years most leaplings celebrate on 28 February or 1 March. Legally, many countries, including the UK, treat 1 March as the day you come of age in a common year. Enter a 29 February birth date above and the page switches to counting your real birthdays.
Questions people ask
Why do we have leap days at all?
The Earth takes about 365.2422 days to go round the Sun from one spring equinox to the next. Without an extra day roughly every four years, the calendar would drift by about a day every four years, and in a few centuries the seasons would be in the wrong months.
Is 2100 a leap year?
No. Century years are only leap years if they divide by 400. So 2000 was a leap year, but 2100, 2200 and 2300 will not be.
How many people are born on 29 February?
About one in every 1,461 births, since the date comes round once every four years. That works out to roughly five million people alive today.
How old is someone born on 29 February 2000?
They count their age normally, 26 in 2026, but they have had only six “real” birthdays on the date itself: 2004, 2008, 2012, 2016, 2020 and 2024.
Sources. Gregorian calendar rules (Inter gravissimas, 1582); tropical year length from the US Naval Observatory. Calculations run in your browser; nothing you enter is sent anywhere.