A year is a leap year if you can divide it by 4 with no remainder, with one exception: a year that divides by 100 only counts if it also divides by 400. That is why 2000 was a leap year, 1900 was not, and 2100 will not be. The next leap year is 2028, and 2026 and 2027 are common years of 365 days.
The rest of this guide shows how to apply the rule, where the 365.2422 figure comes from, and which years are next. It does not cover the history of earlier calendars, only the rule in use today and the arithmetic behind it.
How to tell if a year is a leap year#
Work through three questions in order and stop at the first one that gives an answer.
- Is the year divisible by 4? If not, it is a common year with 365 days. 2026 and 2027 end here.
- Is it divisible by 100? If not, it is a leap year. 2024 and 2028 end here.
- Is it divisible by 400? If yes, it is a leap year. If no, it is not.
| Year | Divisible by 4? | By 100? | By 400? | Leap year? |
|---|---|---|---|---|
| 2026 | No | No | No | No |
| 2024 | Yes | No | No | Yes |
| 1900 | Yes | Yes | No | No |
| 2000 | Yes | Yes | Yes | Yes |
| 2100 | Yes | Yes | No | No |
| 2400 | Yes | Yes | Yes | Yes |
A leap year has 366 days, because February has 29 instead of 28. That is 8,784 hours instead of 8,760 (computed). We also tested the rule against Node.js date handling for every year from 1 to 9999 and found no disagreement.
Why a year needs an extra day#
The seasons do not repeat every 365 days. The BIPM's history of the second records that by 1960 the second was defined as 1/31,556,925.9747 of the tropical year 1900. The tropical year is the time the seasons take to go round once. Divide that many seconds by the 86,400 seconds in a day and you get 365.2422 days, or 365 days, 5 hours, 48 minutes and 46 seconds (computed). The same history notes that an atomic definition of the second replaced this one in 1967.
So a 365-day calendar falls behind the seasons by almost a quarter of a day each year. Adding a day every fourth year nearly cancels that, but not quite, and the leftover builds up. Each step of the rule shrinks it:
| Calendar rule | Average year | Compared with 365.2422 | Slips by one day in about |
|---|---|---|---|
| No leap days | 365 days | 0.2422 days short (about 5 hours 49 minutes) | 4 years |
| Leap day every 4th year | 365.25 days | 0.0078 days long (about 11 minutes) | 128 years |
| Every 4th year, but never in century years | 365.24 days | 0.0022 days short (about 3 minutes) | 455 years |
| Full rule: century years only if divisible by 400 | 365.2425 days | 0.0003 days long (about 26 seconds) | 3,320 years |
The table uses the 1900 figure throughout (all computed). The full rule repeats every 400 years: 97 leap years among 400 years, or 146,097 days in all, which averages 365.2425 days. Of the century years, 1600 and 2000 are leap years but 1700, 1800, 1900, 2100, 2200 and 2300 are not. That is why 1901 to 2000 contains 25 leap years and 2001 to 2100 contains only 24.
Leap years from 2028 to 2072#
Every fourth year from 2028 is a leap year until the century skip. The table adds the weekday of each leap day.
| Leap year | February 29 falls on a |
|---|---|
| 2028 | Tuesday |
| 2032 | Sunday |
| 2036 | Friday |
| 2040 | Wednesday |
| 2044 | Monday |
| 2048 | Saturday |
| 2052 | Thursday |
| 2056 | Tuesday |
The next four are 2060, 2064, 2068 and 2072. After 2096 comes 2104, because 2100 is skipped; 2400 is the next century year that gets a leap day. All of this was computed for this guide.
Leap days repeat their weekday every 28 years, so February 29 is a Tuesday in 2000, 2028 and 2056. That 28-year pattern holds for every year from 1901 to 2099 and is not a rule of the calendar, just a consequence of 28 being 4 times 7.
If you were born on February 29#
Over a full 400-year cycle, February 29 turns up 97 times in 146,097 days, which is once in every 1,506 days or so (computed). Someone born on February 29, 2000 turns 26 in 2026, yet the calendar has shown their date only six times since then: 2004, 2008, 2012, 2016, 2020 and 2024.
What counts as their birthday in a common year is a matter of custom and, for some purposes, of the rules of the authority concerned. This guide does not state any legal rule. If the date has legal force, for a contract, an age limit or an official document, check the official source.
Leap years in software and spreadsheets#
The most common mistake is testing only "divisible by 4". It gives the right answer until 2100, 2200 and 2300, which it wrongly calls leap years (computed). The first wrong answer is 74 years after 2026.
Two more places where the extra day matters:
- Day numbers. March 1 is day 60 of the year in 2026 and day 61 in 2028 (computed).
- Adding a year. One year after February 29, 2028 is a date that does not exist. In JavaScript,
new Date(Date.UTC(2029, 1, 29))rolls over to March 1, 2029. Other tools may choose February 28 instead, so check what yours does before you rely on it.
Our guide to Unix Time, ISO 8601 and UTC: How Computers Store Time covers how computers store dates, and ISO Week Numbers: How to Find the Week Number of Any Date explains the week number that Timelive shows beside the date.
A leap second is not a leap year#
A leap year adds a whole day to the calendar so that dates keep pace with the seasons, and its rule can be computed centuries ahead. A leap second adds one second to the clock at the end of a day so that UTC stays close to the time set by Earth's rotation. It is added when the gap between that rotation time (UT1) and UTC is predicted to approach about 0.9 seconds, and it cannot be computed ahead, because it follows measurements of the planet's spin.
There have been 27 since 1972, all additions, and the latest came at the end of December 31, 2016 (counted from the leap-second list in the tz database, as of October 2026, where the offset between TAI and UTC steps from 10 seconds in 1972 to 37 in 2017). The IERS announced in Bulletin C 72, dated July 6, 2026 and forwarded to the IANA tz mailing list, that none will be added at the end of December 2026. In 2022 the CGPM adopted Resolution 4 on the use and further development of UTC. As generally reported, it allows a larger difference between UT1 and UTC in or before 2035, which would mean fewer or no leap seconds; check the resolution text for its exact wording. How Your Phone Knows the Time: Atomic Clocks, Leap Seconds and the Network Behind Them explains why the clocks need them and what is being decided.
Frequently asked questions
When is the next leap year?
The next leap year is 2028. As of October 9, 2026, 2026 and 2027 are common years of 365 days. The extra day, February 29, 2028, falls on a Tuesday, 508 days from October 9, 2026. A leap year has 366 days.
Is 2100 a leap year?
No. 2100 is divisible by 4 and by 100 but not by 400, so it is a common year with 365 days. 1900 and 2200 are common years for the same reason, and 2000 and 2400 are leap years because they divide by 400.
Why is there a leap year every four years?
A year of seasons is about 365.2422 days, so a 365-day calendar loses almost a quarter of a day each year. A leap day every fourth year brings the calendar back in step, and the century exceptions trim the small surplus. The full rule averages 365.2425 days per year.
How often is someone born on February 29?
February 29 occurs 97 times in every 400 years, or once in about 1,506 days. A person born on February 29, 2000 has had only six leap-day birthdays by October 2026, in 2004, 2008, 2012, 2016, 2020 and 2024. The date used in common years is a matter of custom or local rules.
Is a leap second the same as a leap year?
No. A leap year adds a whole day to the calendar, while a leap second adds one second to the clock at the end of a day. Leap years follow a fixed rule you can compute. Leap seconds depend on measurements of Earth's rotation and are announced only when needed. In 2022 the CGPM decided, as generally reported, to allow a larger UT1 and UTC difference in or before 2035, so leap seconds may be discontinued.
Sources and further reading
- History of the SI second — BIPM
- Resolution 4 of the 27th CGPM (2022): On the use and further development of UTC — BIPM
- Recommendation ITU-R TF.460-6: Standard-frequency and time-signal emissions — International Telecommunication Union
- Fwd: Bulletin C number 72 — IANA tz mailing list (forwarding IERS Bulletin C 72)