Time Unit Converter

How many weeks are in a year

A common year is 52 weeks and 1 day, because 365 divided by 7 is 52.142857 and not a whole number. A leap year is 52 weeks and 2 days. 2026 is a common year, so it has 365 days and 52 weeks and 1 left over.

That leftover day is why your birthday moves forward one weekday each year, and two after a leap year. It is also why "52 weeks" and "a year" are not interchangeable in a contract.

The units that are not a fixed length

Most time units are exact. Sixty seconds make a minute, sixty minutes make an hour, twenty-four hours make a day. Those conversions have one right answer and this tool gives it.

Three units do not work that way, and any converter that pretends otherwise is quietly guessing on your behalf.

A month is 28, 29, 30 or 31 days. There is no such thing as "a month in days" without saying which month. When you convert to or from months, this tool uses an average month of 30.436875 days, which is the mean over a 400-year Gregorian cycle, and it says so under the result. If you need a real month, use the date calculator instead and give it real dates.

A year is 365 or 366 days. The average over the Gregorian cycle is 365.2425 days, which is what this tool uses. That average is itself the whole design of the calendar: 365 days plus a leap day every four years, minus one every hundred, plus one back every four hundred.

A day is almost always 24 hours, and twice a year in most of the world it is 23 or 25 because the clocks change. Very occasionally it is 86,401 seconds because a leap second has been inserted. For converting a duration, 24 hours is the right assumption and this tool uses it. For working out what time something actually happens, use the hours calculator, which handles the real days.

So: exact where exact exists, one clearly stated assumption where it does not, and a pointer to the right tool when you need the real thing. That is the whole design of this page.

Which second are we talking about

For everyday purposes there is one second and this section does not matter. For anything scientific or technical, there are two answers.

The SI second is defined by a caesium atomic transition and does not vary. Atomic clocks count these, and so does International Atomic Time.

The day, meanwhile, is set by the Earth’s rotation, which is not perfectly regular and is gradually slowing. Because 86,400 SI seconds and one rotation of the Earth are not exactly equal, the two drift apart.

Leap seconds exist to close that gap. Coordinated Universal Time has occasionally had an extra second inserted, giving a minute with 61 seconds in it, to keep clock time aligned with the Earth. There is an agreed plan to stop doing this, because a minute that is sometimes 61 seconds long breaks a great deal of software.

This converter uses fixed 60-second minutes throughout. If you are doing work where leap seconds matter, you already know that you need a different tool.

Common conversions

For developers

Some values worth having in one place.

A Unix timestamp counts seconds since 1 January 1970 at 00:00:00 UTC, and it does not count leap seconds. A millisecond timestamp is the same thing times 1,000, which is what JavaScript’s Date.now() returns.

The 32-bit signed limit is 2,147,483,647 seconds, which runs out on 19 January 2038. That is the year 2038 problem, and it is why timestamps should be stored in 64 bits.

Common intervals in seconds: an hour is 3,600, a day 86,400, a week 604,800, thirty days 2,592,000, a 365-day year 31,536,000.

Do not derive a date by adding seconds to a timestamp and hoping. Adding 86,400 seconds moves you forward exactly 24 hours, which is not the same as moving to the same time tomorrow when the clocks change in between. Use a date library, and read the hours calculator page if you want the detail.