How many days are in each month and how does the leap year rule work?

Seven months have 31 days (January, March, May, July, August, October, December), four have 30 days (April, June, September, November), and February has 28 days, or 29 in a leap year. A year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 2024 and 2000 are leap years, while 2025 and 1900 are not.

What is the length of each month?

The table also shows the day of the year on which each month starts, which is useful for counting days manually.

Assumptions: Gregorian calendar. The common-year column uses 2025 and the leap-year column uses 2024. Values come from the day-count function behind Mathify's Days in Month Calculator and the script scripts/content-data/calendar-reference.ts, which also checked that the months sum to 365 and 366 days.

January: 31 days (31 in a leap year) | day of year on the 1st: 1 (1 in a leap year)
February: 28 days (29 in a leap year) | day of year on the 1st: 32 (32 in a leap year)
March: 31 days (31 in a leap year) | day of year on the 1st: 60 (61 in a leap year)
April: 30 days (30 in a leap year) | day of year on the 1st: 91 (92 in a leap year)
May: 31 days (31 in a leap year) | day of year on the 1st: 121 (122 in a leap year)
June: 30 days (30 in a leap year) | day of year on the 1st: 152 (153 in a leap year)
July: 31 days (31 in a leap year) | day of year on the 1st: 182 (183 in a leap year)
August: 31 days (31 in a leap year) | day of year on the 1st: 213 (214 in a leap year)
September: 30 days (30 in a leap year) | day of year on the 1st: 244 (245 in a leap year)
October: 31 days (31 in a leap year) | day of year on the 1st: 274 (275 in a leap year)
November: 30 days (30 in a leap year) | day of year on the 1st: 305 (306 in a leap year)
December: 31 days (31 in a leap year) | day of year on the 1st: 335 (336 in a leap year)

What is the leap year rule?

A year is a leap year when:

It is divisible by 4, and
It is not divisible by 100, unless
It is also divisible by 400.

Examples, checked with the leap-year function in Mathify's repository:

1900: common year (divisible by 100 but not 400), Feb has 28 days, 365 days in the year
2000: leap year (divisible by 400), Feb has 29 days, 366 days in the year
2023: common year (not divisible by 4), Feb has 28 days, 365 days in the year
2024: leap year (divisible by 4 and not by 100), Feb has 29 days, 366 days in the year
2025: common year (not divisible by 4), Feb has 28 days, 365 days in the year
2026: common year (not divisible by 4), Feb has 28 days, 365 days in the year
2028: leap year (divisible by 4 and not by 100), Feb has 29 days, 366 days in the year
2100: common year (divisible by 100 but not 400), Feb has 28 days, 365 days in the year
2400: leap year (divisible by 400), Feb has 29 days, 366 days in the year

Why does the rule skip some century years?

Adding a day every 4 years would make the average year 365.25 days. The century exceptions bring the average down. Over a 400-year cycle there are 97 leap years, giving 146097 days, so the average year is 365.2425 days. That is why the 100 and 400 rules exist, and why 1900 and 2100 are not leap years but 2000 and 2400 are.

Between January 1, 2000 and January 1, 2100 there are 36525 days. That span contains 25 leap years (2000, 2004 and so on up to 2096), because 2100 is not one. Check: 100 x 365 + 25 = 36,525.

How many days are between two dates?

Day counts below were computed from UTC midnight timestamps, so daylight-saving changes cannot add or remove a day. Each count is the number of days from the first date to the second, not counting the first day.

January 1 to December 31, 2025: 364 days
January 1 to December 31, 2024: 365 days
January 1, 2025 to January 1, 2026: 365 days
January 1, 2024 to January 1, 2025: 366 days
February 28 to March 1, 2025: 1 day
February 28 to March 1, 2024: 2 days
March 1 to June 1, 2026: 92 days
June 1 to September 1, 2026: 92 days
September 1 to December 1, 2026: 91 days
October 15, 2026 to December 25, 2026: 71 days
October 15, 2026 to January 1, 2027: 78 days
January 1, 2000 to January 1, 2100: 36525 days

Rule of thumb: end date minus start date, then add 1 if you want to count both the first and the last day. October 15 to December 25, 2026 is 71 days apart, which is 10 weeks and 1 day, or 72 days if both days are counted.

Worked example: how long until the end of the year?

From October 15, 2026 to January 1, 2027 is 78 days. In weeks that is 11 weeks and 1 day. For specific cases such as business days only or calendar duration in years, months and days, use the Date Difference Calculator, the Duration Calculator or the Business Days Calculator. To count how old someone is in days, see the Age in Days Calculator.

Why do 52 weeks not equal a year?

52 weeks is 364 days. A common year has 365 days, so it is 1 day longer, and a leap year is 2 days longer. Over time those extra days push dates onto different weekdays, which is why the same date falls on a different weekday each year. The Day of Week Calculator will tell you the weekday for any date.

Common mistakes

Treating every century year as a leap year.
Forgetting that the end date is included or excluded depending on the question.
Counting February as 28 days in a leap year.
Mixing up inclusive and exclusive day counts in contracts and deadlines.
Using local time instead of dates, so a daylight-saving shift changes the count.

Key takeaways

31-day months: 7. 30-day months: 4. February is the exception.
Leap year: divisible by 4, not by 100 unless also divisible by 400.
A 400-year cycle has 97 leap years and 146,097 days.
For exact date arithmetic use a calculator.

FAQ

Is 2100 a leap year?

No. It is divisible by 100 but not by 400, so February 2100 has 28 days.

Is 2000 a leap year?

Yes. It is divisible by 400.

How many days are in a leap year?

366.

How many days are there between January 1 and December 31?

364 days apart in a common year and 365 in a leap year. Counting both days gives 365 and 366.

What day of the year is March 1?

Day 60 in a common year and day 61 in a leap year.

How many days are in each month and how does the leap year rule work?