Time conversions are exact right up to the month, at which point they stop being conversions at all and become approximations. Seconds, minutes, hours, days and weeks are fixed multiples. Months and years are not.
Quick reference
The table follows whichever units you select above.
The exact part
1 min = 60 s1 h = 3,600 s1 d = 86,400 s1 wk = 604,800 s
The sixties come from Babylonian base-60 arithmetic, which survived into modern timekeeping because 60 divides evenly by 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30 — unusually convenient for a civilisation without decimals.
The part that is an average
A month is anywhere from 28 to 31 days, so any single figure is a convention. This converter uses the average Gregorian month, 30.436875 days, which is a year divided by twelve. A year here is 365.25 days, the Julian year used in astronomy.
Do not use averaged months for contracts or billing. "Three months" in an agreement means three calendar months from a date, not 91.3 days. The two diverge, and the difference has ended up in court more than once. Use a date calculator, not a unit conversion.
Which year?
| Year | Days | Used for |
|---|---|---|
| Common | 365 | Ordinary calendar years |
| Leap | 366 | Most years divisible by four |
| Julian | 365.25 | Astronomy, light-years |
| Gregorian average | 365.2425 | Long-run calendar average |
| Tropical | 365.24219 | Equinox to equinox |
Leap seconds
The Earth's rotation is gradually slowing and does so unevenly, so leap seconds have occasionally been inserted to keep clocks aligned with the planet. Twenty-seven have been added since 1972. In 2022 the international metrology community voted to abandon the practice by 2035, because leap seconds cause more trouble for computer systems than the drift they correct.
Common questions
How many seconds are in a day?
86,400, by definition. Rare leap seconds aside, this is exact.
How many weeks are in a month?
About 4.35 on average, not 4. The difference is why twelve four-week periods come to 336 days rather than 365, and why monthly and four-weekly pay cycles drift apart across a year.
How many hours are in a year?
8,760 for a 365-day year, or 8,784 in a leap year.