Generally speaking, a calendar year begins on the New Years Day of the given calendar system and ends on the day before the following New Years Day, and thus consists of a whole number of days. A year can also be measured by starting on any other named day of the calendar, and ending on the day before this named day in the following year. This may be termed a "years time", but not a "calendar year". To reconcilie the calendar year with the astronomical cycle certain years contain extra days. The Gregorian year, which is in use in most of the world, begins on January 1 and ends on December 31. It has a length of 365 days in an ordinary year, with 8.760 hours, 525.600 minutes, or 31.536.000 seconds; but 366 days in a leap year, with 8.784 hours, 527.040 minutes, or 31.622.400 seconds. With 97 leap years every 400 years, the year has an average length of 365.2425 days. Other formula-based calendars can have lengths which are further out of step with the solar cycle: for example, the Julian calendar has an average length of 365.25 days, and the Hebrew calendar has an average length of 365.2468 days. The Islamic calendar is a lunar calendar consisting of 12 months in a year of 354 or 355 days. The astronomers 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 the astronomers value changes over time, so John Herschels suggested correction to the Gregorian calendar may become unnecessary by the year 4000.
A common year is a calendar year with 365 days, as distinguished from a leap year, which has 366. More generally, a common year is one without intercalation. The Gregorian calendar employs both common years and leap years to keep the calendar aligned with the tropical year, which does not contain an exact number of days. The common year of 365 days has 52 weeks and one day, hence a common year always begins and ends on the same day of the week for example, January 1 and December 31 fell on a Friday in 2021 and the year following a common year will start on the subsequent day of the week. In common years, February has four weeks, so March will begin on the same day of the week. November will also begin on this day. In the Gregorian calendar, 303 of every 400 years are common years. By comparison, in the Julian calendar, 300 out of every 400 years are common years, and in the Revised Julian calendar used by Greece 682 out of every 900 years are common years.
A leap year is a calendar year that contains an additional day added to keep the calendar year synchronized with the astronomical year or seasonal year. Because astronomical events and seasons do not repeat in a whole number of days, calendars that have the same number of days in each year drift over time with respect to the event that the year is supposed to track. By inserting an additional day or month into the year, the drift can be corrected. A year that is not a leap year is a common year. For example, in the Gregorian calendar, each leap year has 366 days instead of 365, by extending February to 29 days rather than the common 28. These extra days occur in each year which is an integer multiple of 4 except for years evenly divisible by 100, which are not leap years unless evenly divisible by 400. Similarly, in the lunisolar Hebrew calendar, Adar Aleph, a 13th lunar month, is added seven times every 19 years to the twelve lunar months in its common years to keep its calendar year from drifting through the seasons. In the Bahai Calendar, a leap day is added when needed to ensure that the following year begins on the March equinox. The term leap year probably comes from the fact that a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, but the day of the week in the 12 months following the leap day from March 1 through February 28 of the following year will advance two days due to the extra day, thus leaping over one day in the week. For example, Christmas Day December 25 fell on a Tuesday in 2012, Wednesday in 2013, Thursday in 2014, and Friday in 2015, but then leapt over Saturday to fall on a Sunday in 2016. The length of a day is also occasionally corrected by inserting a leap second into Coordinated Universal Time UTC because of variations in Earths rotation period. Unlike leap days, leap seconds are not introduced on a regular schedule because variations in the length of the day are not entirely predictable. Leap years can present a problem in computing, known as the leap year bug, when a year is not correctly identified as a leap year or when February 29 is not handled correctly in logic that accepts or manipulates dates.
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