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Units of Time
Time is measured in a ladder of units, from the vibrations of atoms to spans longer than any human life. The second is the SI base unit — everything else is defined from it.
| Unit | Equals | Notes |
|---|---|---|
| Nanosecond (ns) | 10⁻⁹ s | Light travels ~30 cm in 1 ns; CPU clock ticks are fractions of a ns. |
| Microsecond (µs) | 10⁻⁶ s | Typical scale of high-speed electronics and camera flashes. |
| Millisecond (ms) | 10⁻³ s | The unit of JavaScript timestamps and human reaction times (~200 ms). |
| Second (s) | SI base unit | Defined by 9,192,631,770 vibrations of the caesium-133 atom. |
| Minute (min) | 60 s | From Latin pars minuta prima, the "first small part" of an hour. |
| Hour (h) | 60 min = 3,600 s | 24 per day — a convention inherited from ancient Egypt. |
| Day | 24 h = 86,400 s | One rotation of Earth relative to the sun (mean solar day). |
| Week | 7 days = 604,800 s | Not astronomical — a cultural unit from the Babylonian and Jewish traditions. |
| Fortnight | 14 days | "Fourteen nights" — still common in British and Indian English. |
| Month | 28–31 days | Loosely one lunar cycle (29.53 days); calendar months vary. |
| Quarter | 3 months | The standard business reporting period. |
| Year | 365 or 366 days | Averages 365.2425 days in the Gregorian calendar — hence leap years. |
| Olympiad | 4 years | The interval between Olympic Games in ancient Greece. |
| Lustrum | 5 years | From the Roman census-and-purification cycle. |
| Decade | 10 years | From Greek deka, ten. |
| Score | 20 years | "Four score and seven years ago" = 87 years. |
| Generation | ~25–30 years | A social measure, not a fixed unit. |
| Jubilee | 50 years | From the biblical fiftieth-year celebration. |
| Century | 100 years | From Latin centum, hundred. |
| Millennium | 1,000 years | From Latin mille, thousand. |
Curious units
- Jiffy — informally "a very short time"; in computing, one tick of the system timer (often 1/60 or 1/100 s); in physics, the time light takes to travel one centimetre.
- Shake — 10 nanoseconds, used in nuclear physics ("two shakes of a lamb's tail").
- Sidereal day — Earth's rotation measured against the stars: 23 h 56 m 4 s, about 4 minutes shorter than the solar day.
- Sol — a Martian day, 24 h 39 m 35 s, used by Mars mission teams.
Need to convert between units? Try the seconds, minutes and milliseconds converters, or the date difference calculators.