The calendar divides the year into months, weeks, and days and is the method of ordering the years. From year 1, an assumed date of the birth of Jesus, dates are calculated backwards (BC `before Christ' or BCE `before common era') and forwards (AD, Latin anno Domini `in the year of the Lord'). The lunar month (the period between one new moon and the next) averages 29.5 days, but the Western calendar uses for convenience a calendar month with a complete number of days, 30 or 31 (Feb has 28). For adjustments, since there are slightly fewer than six extra hours a year left over, they are added to Feb as a 29th day every fourth year (leap year), century years being excepted unless they are divisible by 400. For example, 1996 was a leap year; and 2000 is a leap year as it is divisible by 400.

The month names are derived as follows: January from Janus, Roman god; February from Februar, Roman festival of purification; March from Mars, Roman god; April from Latin aperire, `to open'; May from Maia, Roman goddess; June from Juno, Roman goddess; July from Julius Caesar, Roman general; August from Augustus, Roman emperor; September, October, November, December (originally the seventh-tenth months) from the Latin words meaning seventh, eighth, ninth, and tenth, respectively.

The days of the week are Monday named after the Moon; Tuesday from Tiu or Tyr, Anglo-Saxon and Norse god; Wednesday from Woden or Odin, Norse god; Thursday from Thor, Norse god; Friday from Freya, Norse goddess; Saturday from Saturn, Roman god; and Sunday named after the Sun. All early calendars except the ancient Egyptian were lunar.

The Western or Gregorian calendar derives from the Julian calendar instituted by Julius Caesar 46 BC. It was adjusted by Pope Gregory XIII 1582, who eliminated the accumulated error caused by a faulty calculation of the length of a year and avoided its recurrence by restricting century leap years to those divisible by 400. Other states only gradually changed from Old Style to New Style; Britain and its colonies adopted the Gregorian calendar 1752, when the error amounted to 11 days, and 3 Sept 1752 became 14 Sept (at the same time the beginning of the year was put back from 25 March to 1 Jan). Russia did not adopt it until the October Revolution of 1917, so that the event (then 25 Oct) is currently celebrated 7 Nov.