Time Unit Converter

Which units here are exact

A minute is exactly 60 seconds, an hour exactly 3600, a day exactly 86,400 and a week exactly 604,800. Those are definitional and never vary — with one exception: the occasional leap second inserted into UTC makes a particular day 86,401 seconds long. Leap seconds are scheduled to be discontinued by 2035.

Why a month has no fixed length

Months run from 28 to 31 days. The average used here is the Gregorian calendar's own average: 365.2425 days per year divided by 12, giving 30.436875 days. That is the right figure for long-run averages such as “how many months is 10,000 hours”, and the wrong one for anything involving specific dates. If you need the gap between two actual dates, count calendar days rather than dividing.

Three different years

The Gregorian average year is 365.2425 days — the calendar's own long-run average once leap-year rules are applied. The Julian year is exactly 365.25 days and is the definition astronomers use for the light-year. The common year is a plain 365 days, which is what people usually mean in casual arithmetic. All three are listed separately above because the choice changes the answer by up to a quarter of a percent — which over a century is most of a year.

The leap-year rule, in full

A year is a leap year if it is divisible by 4, except years divisible by 100, except years divisible by 400. So 1900 was not a leap year and 2000 was. That exception-to-the-exception is the single most common date bug in software, and it is why 365.2425 rather than 365.25 is the correct civil average.

Frequently Asked Questions