Time units and their equivalents
| Unit | Symbol | In seconds |
|---|---|---|
| Second | s | 1 sbase unit |
| Millisecond | ms | 0.001 s |
| Microsecond | µs | 0.000001 s |
| Minute | min | 60 s |
| Hour | h | 3,600 s |
| Day | d | 86,400 s |
| Week | wk | 604,800 s |
| Month (average) | mo | 2,629,744 s |
| Year (average) | yr | 31,556,925 s |
Frequently asked questions
How many days in a month, for a conversion?
30.436875 days, the Gregorian calendar average: 365.2425 days divided by twelve. It is the only defensible figure, since real months hold 28, 29, 30 or 31 days. For contractual or financial work, check the convention in force, often exactly 30 days.
Why isn't a year 365 days?
Because the Earth takes about 365.2425 days to complete its orbit. Leap years make up that quarter day, with the exception of centuries not divisible by 400: 1900 was not a leap year, 2000 was. Our conversions use this average, called the Gregorian year.
Is the second defined by the Earth?
Not since 1967. It equals 9,192,631,770 periods of the transition between two levels of the caesium 133 atom. The Earth's rotation is slowing slightly, which is why a leap second is occasionally added to keep civil time aligned with the sky.
How many milliseconds are in a second?
A thousand, which is a million microseconds and a billion nanoseconds. These units are not confined to laboratories: a 60 Hz screen draws a frame every 16.67 ms, a 1/1000 s shutter speed lasts exactly one millisecond, and a blink takes about a hundred. At the nanosecond scale light travels only thirty centimetres, which is why cable lengths are counted in fast circuits.
Why does a date turn into a large number?
Because a machine stores time as a duration elapsed since an origin. Unix time counts seconds from 1 January 1970 at 00:00 UTC, leap seconds excluded, and many languages count the same in milliseconds; a signed 32-bit counter overflows on 19 January 2038 at 03:14:07 UTC. A spreadsheet counts days instead: 45678 is 21 January 2025, and the decimal part carries the time of day, 0.5 for noon.
The month and the year have no fixed length
The second is the only unit of time the SI defines: 9,192,631,770 periods of the transition between two hyperfine levels of the caesium-133 atom, since 1967. Everything else is convention. The minute, the hour and the day are accepted for use with the SI because custom imposes them, not because they follow from it. The month and the year change length from one instance to the next: February has twenty-eight days or twenty-nine, the year three hundred and sixty-five or three hundred and sixty-six. This converter therefore uses average lengths, 365.25 days for the year and one twelfth of that, 30.4375 days, for the month.
The terrestrial day, for its part, is getting longer. The Earth's rotation is slowing under tidal friction, and twenty-seven leap seconds were added to Coordinated Universal Time between 1972 and 2016 to keep it aligned with the sky. Those extra seconds disrupt computer systems, which never expect a minute of sixty-one seconds. In 2022 the General Conference on Weights and Measures decided to abandon them by 2035 at the latest: clocks will then follow caesium alone and let the sky drift.