**Definitions and Standards**

Unit | Size | Notes |
---|---|---|

yoctosecond | 10−24 s | |

zeptosecond | 10−21 s | |

attosecond | 10−18 s | shortest time uncertainty in present measurements |

femtosecond | 10−15 s | pulse time of ultrafast lasers (100 as = 0.1 fs) |

picosecond | 10−12 s | |

nanosecond | 10−9 s | time for molecules to fluoresce |

microsecond | 10−6 s | |

millisecond | 0.001 s | |

second | 1 s | SI base unit |

minute | 60 seconds | |

hour | 60 minutes | |

day | 24 hours | |

week | 7 days | Also called sennight |

fortnight | 14 days | 2 weeks |

lunar month | 27.2–29.5 days | Various definitions exist. |

month | 28–31 days | |

quarter | 3 months | |

year | 12 months | |

common year | 365 days | 52 weeks + 1-day |

leap year | 366 days | 52 weeks + 2 days |

tropical year | 365.24219 days | average |

Gregorian year | 365.2425 days | average |

Olympiad | 4-year cycle | |

lustrum | 5 years | Also called pentad |

decade | 10 years | |

Indiction | 15-year cycle | |

generation | 17–35 years | approximate |

jubilee (Biblical) | 50 years | |

century | 100 years | |

millennium | 1,000 years | |

exasecond | 1018 s | |

cosmological decade | varies | |

cosmological century | varies |

The SI base unit for time is the SI second. From the second, larger units such as the minute, hour and day are defined, though they are "non-SI" units because they do not use the decimal system, and also because of the occasional need for a leap second. They are, however, officially accepted for use *with* the International System. There are no fixed ratios between seconds and months or years as months and years have significant variations in length.

The official SI definition of the second is as follows:

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

At its 1997 meeting, the CIPM affirmed that this definition refers to a caesium atom in its ground state at a temperature of 0 K. Previous to 1967, the second was defined as:

the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.

The current definition of the second, coupled with the current definition of the metre, is based on the special theory of relativity, which affirms our space-time to be a Minkowski space.

Read more about this topic: Time

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