Date Calculator

Two counting conventions, one of them on a switch

The number of days between 1 January and 31 December is 364, or 365 if the first day counts as well. Neither is wrong: a timesheet counts both ends of a shift, a hotel counts the nights. The switch says which convention the headline number uses, and the other number is printed underneath so nobody has to guess.

Weekdays and weekend days are counted under the same convention, so they always add up to the total. Friday to Monday with the end date counted is four days: two weekdays and two weekend days. The same pair without the end date is three days: one weekday and two weekend days.

Public holidays are deliberately not subtracted. There is no single list of them - they differ by country, by state, by year, and by whether a substitute day was granted - so a page that guessed would be wrong on precisely the day it mattered. If you need working days rather than weekdays, take the weekday count and subtract your own holiday list.

Months and years are stepped, not multiplied

A month is not a number of days: 28, 29, 30 or 31, depending on the month and the year. Multiplying by 30.44 - the average month - gives answers that are close and never exactly the date somebody meant. This page steps through the calendar instead, so three months from 15 March is 15 June, and three months from 31 January is 30 April, because the 31st does not exist in February or April.

That clamping is the same rule the calendar span uses, which is why the two are exact inverses: if the page says two dates are one month apart, adding one month to the first gives the second, every time. A span shown as "13 months" is also shown as "1 year 1 month", and a span that printed "1 month 30 days" for the same pair would mean the two halves of the calculation disagreed with each other.

Years behave the same way around leap days: 29 February 2024 plus one year is 28 February 2025, because 2025 has no 29th of February. Four years later it is 29 February 2028 again.

Leap years, and the rule people forget

A year is a leap year if it is divisible by 4, unless it is also divisible by 100, unless it is also divisible by 400. So 2024 was one, 1900 was not, and 2000 was. The rule keeps the calendar aligned with the seasons: the average Gregorian year is 365.2425 days against a tropical year of about 365.2422, a drift of roughly one day in 3200 years.

Every function here uses the full rule rather than the common shortcut of "divisible by 4", which gets 1900 wrong by a day. The day-of-the-year figure on the page prints the year length it counted against - 366 in a leap year, 365 otherwise - so the difference is visible rather than assumed.

Weeks follow ISO 8601, which is not "week 1 is January"

An ISO week runs from Monday to Sunday, and week 1 is the week containing the first Thursday of the year. The consequence is that a year has 52 or 53 weeks, and that the first days of January can belong to the last week of the previous year: 1 January 2027 is a Friday, so it is week 53 of 2026. The page prints the week with its week-year (2026-W53) so the number and the year are stated together.

This is the convention behind week numbers in Europe, in payroll systems and in `%G-W%V` date formatting. It is not the same as "the 38th week of the year", which is what dividing the day of the year by seven gives, and the difference shows up in the first and last weeks of every year.