Guide

Time units and the length of a year

The second is the only time unit with a physical definition. Every larger unit is a defined multiple — with the exception of month and year, which follow the calendar and therefore have no fixed number of seconds.

The second as base unit

The second is an SI base unit and is defined today via the oscillation of a caesium atom. A minute has 60 seconds, an hour 60 minutes, a day 24 hours. Day, hour and minute are therefore fixed exactly: a day has precisely 86,400 seconds.

Why week and month are different

The week is pure convention and has exactly 7 days, that is 604,800 seconds. The month, by contrast, follows the lunar cycle and the calendar: it has between 28 and 31 days. A fixed length in seconds cannot be given for the month.

The Julian year as a calculation unit

A calendar year has 365 or 366 days. Technical and astronomical calculations therefore use the Julian year of exactly 365.25 days, that is 31,557,600 seconds. It is an average, not a calendar figure: a single calendar year never has 365.25 days.

Leap years and the exception at the century

Every four years a leap day is inserted to compensate for the deviation from the solar year. The rule has an exception, however: years divisible by 100 are not leap years unless they are also divisible by 400. That is why 1900 was not a leap year while 2000 was. A span of 100 years therefore contains 36,524 or 36,525 days.

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Unit family: Time

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