Time Unit Converter

About the Time Unit Converter

Convert a length of time between picoseconds, milliseconds, seconds, minutes, hours, days, weeks, fortnights, average months, years, decades, centuries and millennia. Type in either box and the other updates; the table underneath shows the same duration in every unit, and the panel under the result writes it as days, hours, minutes and seconds (100,000 s = 1 d 3 h 46 min 40 s), as an ISO 8601 duration (P1DT3H46M40S) and as h:mm:ss (27:46:40).

Every unit is a fixed number of seconds and the arithmetic uses exact fractions, so 90 minutes is exactly 1.5 hours and a Gregorian year is exactly 31,556,952 seconds. Years are where converters disagree, so all of them are listed and labelled: the 365-day common year, the 366-day leap year, the Gregorian average of 365.2425 days and the Julian year of 365.25 days used in astronomy.

This page converts durations, not clock times: to see the time in another city use the Time Zone Converter, and to add days or months to a date use the Date Calculator.

Common uses

• Turning a number of seconds from a log file, a stopwatch or a database into days, hours and minutes.
• Reading and writing ISO 8601 durations (PT1H30M) for APIs, calendar files and video metadata.
• Timeouts and latencies in milliseconds, microseconds and nanoseconds.
• Rough planning in weeks, months and years — and knowing which year length a figure uses.
• Homework on unit conversion, with an exact formula for every pair of units.

Which year and which month?

A year is not one fixed length, so the converter lists four and labels each one:

• Year (365 days) — a common calendar year: 31,536,000 s. This is the year NIST lists in its conversion tables.
• Leap year (366 days) — 31,622,400 s.
• Gregorian year (365.2425 days) — the average over the 400-year cycle of the Gregorian calendar, which has 97 leap years: (400 × 365 + 97) ÷ 400 = 365.2425 days = 31,556,952 s.
• Julian year (365.25 days) — 31,557,600 s, the year astronomers use. A light-year is the distance light travels in one Julian year, and a Julian century is 36,525 days.

An average month is one twelfth of a Gregorian year, 30.436875 days; real months have 28 to 31 days. Decades, centuries and millennia are 10, 100 and 1,000 Gregorian years, and the Julian century is listed separately.

Where the definitions come from

The second is the SI base unit, defined by fixing the caesium-133 frequency at 9,192,631,770 Hz, and the minute (60 s), hour (3,600 s) and day (86,400 s) are listed in Table 8 of the BIPM SI Brochure (9th edition, version 4.01). The 365-day year and the Julian century of 36,525 days are from NIST SP 811, Appendix B.

A day here is always 86,400 s. A UTC day with a leap second has 86,401 s, and on the day a clock changes for daylight saving the local day has 23 or 25 hours — those are calendar questions, not unit conversions.

ISO 8601 durations

ISO 8601 writes a duration as P followed by its parts: P1DT3H46M40S is 1 day, 3 hours, 46 minutes and 40 seconds. The T separates the date part from the time part, so PT5M is 5 minutes but P5M would be 5 months. Weeks are written on their own, as P2W.

The converter writes durations with days, hours, minutes and seconds only, because years and months have no fixed length (P1M is 28 to 31 days, depending on the month it is added to); for the same reason it does not read durations that contain years or months. A fraction of a second goes on the seconds (PT0.5S), and a negative duration gets a leading minus sign (-PT5M), as XML Schema writes it. The output is valid for XML Schema’s duration type and for most programming libraries.

Typing durations

Quick convert understands more than one number and one unit:

• compound durations: 1 h 30 min, 1d 3h 46m 40s, 2 days, 4 hours and 5 minutes, and 1h30 or 2 min 30 (a bare number after hours means minutes, after minutes seconds)
• clock-style durations: 1:30:00 (hours:minutes:seconds) and 1:03:46:40 (days:hours:minutes:seconds)
• ISO 8601 durations: PT1H30M, P2W, P1DT12H, PT1.5H

Add to and a unit to choose the result, as in 1:30:00 to min. A two-part time such as 1:30 is refused because it could mean 1 hour 30 minutes or 1 minute 30 seconds.

Limitations

• Calendar arithmetic — adding a month to 31 January, or counting the days between two dates — depends on the calendar; use the Date Calculator for that.
• Days are exactly 86,400 s: leap seconds and daylight-saving changes are not modelled.
• Sidereal and tropical years, lunar months and other astronomical periods are not included, because they come from the motion of the Earth and the Moon rather than from the second.
• The breakdown, ISO 8601 and h:mm:ss forms are rounded to the nearest picosecond (marked ≈); very large or very small results in the table are shown in scientific notation.

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