Definitions

366 days

Calendar year

According to the Gregorian calendar, the calendar year begins on January 1 and ends on December 31.

Generally speaking, a calendar year begins on the New Year's day of the given calendar system and ends on the day before the following New Year's day. In the Gregorian calendar, this is normally 365 days, but 366 days in a leap year, giving an average length of 365.2425 days.

By convention, a calendar year consists of a natural number of days. To reconcile the calendar year with an astronomical cycle (which could not possibly be reckoned in a whole number of days), certain years contain extra days. Through further insertion of non-day units of time, the calendar year can be further synchronized; however, these extra units of time are not considered part of the calendar.

Length

In the Gregorian calendar, the length of a calendar year is normally 365 days, but 366 days in a leap year giving an average length of 365.2425 days. This is very close to the average length of the Islamic calendar (based on the average time between successive vernal equinoxes, currently 365.2424 days and increasing marginally). Other formula-based calendars can have lengths which are further out of step with the solar cycle, for example, the Julian calendar had an average length of 365.25 days, and the Hebrew calendar has an average length of 365.2468 days.
The astronomer's mean tropical year which is averaged over equinoxes and solstices is currently 365.24219 days, slightly shorter than the average length of the year in most calendars, but astronomer's value changes over time, so William Herschel's suggested correction to the Gregorian calendar may become unnecessary by the year 4000.

Subdivisions

Quarters

The calendar year can be divided into 4 quarters, often abbreviated Q1, Q2, Q3 and Q4.

  • First quarter: from the beginning of January to the end of March
  • Second quarter: from the beginning of April to the end of June
  • Third quarter: from the beginning of July to the end of September
  • Fourth quarter: from the beginning of October to the end of December

See also

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