How Leap Years Work

A common year has 365 days. A leap year has 366, with the extra day added as February 29. The rule for which years get that extra day looks simple at first, every four years, but it has two exceptions that matter for anyone working with dates across long periods. This guide explains where the rule comes from, how to apply it, and what it changes when you count days, add months or years, or work out an age.

Why the calendar needs leap years

A calendar year is meant to keep pace with the seasons, which follow Earth’s orbit around the Sun. That orbit doesn’t take a whole number of days. According to NASA, it takes approximately 365 days and 6 hours. A 365-day calendar falls behind by about a quarter of a day every year.

A quarter of a day sounds trivial, but it adds up. After four years the calendar is about a day behind the seasons, after a century it is roughly 24 days behind, and over several centuries the months would drift through the seasons entirely. Adding one day roughly every four years catches the calendar up.

From the Julian to the Gregorian calendar

The Julian calendar, introduced under Julius Caesar, added a leap day every four years without exception. That gives an average year of exactly 365.25 days.

The trouble is that the year the seasons follow, the tropical year, is a little shorter than that, at about 365.2422 days. The Julian year was too long by about 11 minutes. Over many centuries those minutes built up into days, and by the 16th century the spring equinox had moved well away from its traditional date of March 21.

The Gregorian reform of 1582 fixed this in two ways. First, it removed the days that had built up: in the countries that adopted it immediately, Thursday, October 4, 1582 was followed by Friday, October 15, 1582. Second, it changed the leap year rule so the drift would be much smaller in future. Countries switched at different times. Britain and its colonies, including those in North America, made the change in 1752 under an Act of Parliament passed in 1751. Historical dates before a country’s switch may be recorded in the older calendar, which is worth knowing if you are counting days across those centuries.

The Gregorian leap year rule

NASA’s summary of the rule is the clearest: every year exactly divisible by 4 is a leap year, except years exactly divisible by 100, which are leap years only if they are also exactly divisible by 400.

As three checks, in order:

1. Is the year divisible by 400? If so, it is a leap year. 2000 and 2400 are leap years.
2. If not, is it divisible by 100? If so, it is a common year. 1900, 2100, 2200 and 2300 are not leap years.
3. If not, is it divisible by 4? If so, it is a leap year. 2024 and 2028 are. Otherwise, as with 2026 and 2027, it is a common year.

Dropping three leap days every 400 years leaves 97 leap years in each 400-year cycle. The average year becomes 365 + 97 ÷ 400 = 365.2425 days, which NASA describes as a close approximation to the tropical year. The remaining error is small enough that it won’t matter for everyday date arithmetic for a very long time.

Leap years from 2024 to 2060

Leap years	Next century check
2024, 2028, 2032, 2036, 2040	2100 is not a leap year
2044, 2048, 2052, 2056, 2060	2400 is a leap year

Between 1901 and 2099 the simple “divisible by 4” rule happens to give the right answer every time, because 2000 is divisible by 400. That is why the century exception is easy to forget and why software written with only the four-year rule can go wrong in 2100.

A calendar that repeats every 400 years

A 400-year Gregorian cycle contains 400 × 365 + 97 = 146,097 days. That is exactly 20,871 weeks, so the whole calendar, including the day of the week for every date, repeats every 400 years. October 4, 2426 will fall on the same day of the week as October 4, 2026.

How leap years affect date calculations

Counting days

Any range that includes February 29 has one more day than the same range in a common year. A full leap year has 366 days, and the period from November 1, 2027 to March 15, 2028 is 135 days because it contains February 29, 2028. The days between dates calculator counts real calendar days, so leap days are included automatically. If you estimate with 365 days per year, expect to be out by about a day for every four years in the range.

Adding months and years

Adding a year to February 29 is the awkward case, because the target year usually has no February 29. The date calculator moves the result to the last day of the month, so February 29, 2028 plus 1 year is Wednesday, February 28, 2029. Adding 4 years lands on February 29 again, giving Sunday, February 29, 2032.

The century rule can break the four-year pattern. February 29, 2096 plus 4 years would be February 29, 2100, which doesn’t exist, so the result is February 28, 2100.

Other systems use other conventions. Some move a missing February 29 forward to March 1 instead. Neither choice is wrong, but you should know which one a contract, a piece of software or an official rule uses before relying on a date.

Days of the week

Because 365 days is 52 weeks plus 1 day, a date normally moves forward one weekday each year. A leap day adds a second step. October 4 was a Sunday in 2026, is a Monday in 2027, jumps to a Wednesday in 2028 because February 29, 2028 falls in between, and is a Thursday in 2029.

Daily rates

When an annual amount is divided by the number of days in a year, a leap year has one more day to spread it over. Interest, rent and salary calculations handle this in different ways, and some always use 365 days, so check the convention a lender or employer uses. The simple interest calculator is a good place to see how the length of a period changes the result.

Leap-day birthdays

People born on February 29 have an actual birthday only in leap years. Someone born on February 29, 2000 had real February 29 birthdays in 2004, 2008, 2012, 2016, 2020 and 2024, six in total by 2026.

In common years, there are two reasonable answers for when their age goes up: February 28, the last day of February, or March 1, the day after February 28. The age calculator uses February 28, which matches how it handles other month-end dates. On that convention, a person born on February 29, 2004 is 21 years, 11 months and 29 days old on February 27, 2026 and turns 22 on February 28, 2026.

Legal rules vary. Some jurisdictions treat March 1 as the date a leap-day baby reaches a legal age in a common year, and the difference can matter for driving, voting or buying alcohol. If a single day matters, check the rule where you live rather than relying on any calculator’s convention.

Leap years and leap seconds

A leap second is a different adjustment. Leap years keep the calendar in step with Earth’s orbit around the Sun. Leap seconds are occasional one-second adjustments to clock time to keep it close to Earth’s rotation. They don’t change the date, and calendar-date calculators such as those on this site are not affected by them.

Common mistakes

• Using only the four-year rule. It works from 1901 to 2099 but fails in 2100, 2200 and 2300.
• Assuming a year is always 365 days. Use calendar dates, not 365-day multiples, when an exact count matters.
• Assuming February 29 plus one year has an obvious answer. Different systems use February 28 or March 1. Check which one applies.
• Comparing historical dates across calendars. Dates before a country adopted the Gregorian calendar may be in the Julian calendar, so day counts across that boundary need care.

Summary

Leap years exist because Earth’s orbit takes about a quarter of a day longer than 365 days. The Gregorian rule adds February 29 in years divisible by 4, skips it in century years, and puts it back in years divisible by 400. For everyday date work, the main effects are an extra day in some ranges, a February 29 that sometimes has no match in the target year, and birthdays that need a convention. The time and date calculators apply the rule automatically, so you can check any of these cases directly.

Sources

1. Calendars and their History — NASA Goddard Space Flight Center. Checked October 4, 2026.
2. What Is a Leap Year? — NASA Space Place. Checked October 4, 2026.
3. A Walk Through Time — Ancient Calendars — National Institute of Standards and Technology. Checked October 4, 2026.