All calendars are based to some extent on the tropical or Solar Year. This is the time taken for the Earth to complete one orbit of the sun: 365.2564 days. The international standard calendar is the Gregorian calendar.
Unless otherwise stated, the following dates are all Gregorian.
The Gregorian calendar is a development of the Julian Calendar (introduced in 46BC) which is based on a year of 365.25 days. The slight misalignment between the length of the solar and Julian year was, by 1582, 10 days. In 1582 Pope Gregory XIII ordered that October 5 now become October 15 to remove this misalignment. The British Isles changed to the Gregorian Calendar in 1752. The late adoption resulted in the loss of 11 days.
The Christian era began with the birth of Christ (0AD) although evidence suggests that he was actually born in 4BC. The calendar is Gregorian and was introduced into England by St. Augustine in AD597. The Council of Chelsea ordered its use throughout England in AD816.
The Jewish Calendar was devised in AD358 by Rabbi Hillel II. Based on the lunar cycle the Jewish calendar is calculated from 3761BC and has 12 alternate months of 30 and 29 days. To keep it roughly in line with the solar year a thirteenth month is occasionally added. The years are prefixed AM- 'anno mundi', in the year of the world (1996AD = AM5757).
The Islamic calendar is used in Iran, Turkey, Arabia and Egypt. Its epoch is the flight of Mohammed from Mecca in AD622 (The Hejira). The years are purely lunar and run in 30 year cycles each with 12 months of 29 or 30 days.
The Chinese calendar is purely lunar. It has a sixty year cycle with the new year beginning at the first new moon after the sun enters Aquarius (between 21 Jan and 19 Feb). It is still in used in China even though the official calendar has been Gregorian since 1911.
Used in Egypt and Ethiopia, the Coptic calendar has twelve months of 30 days plus 5 complementary days (6 every fourth year). The Coptic era began in AD284.
Before 1752 in England the Historical Year began on the 1st of January but the Civil or Legal Year began on the 25th of March. See also Gregorian calendar.
Introduced in September 1792 following the French Revolution and discontinued in 1806. A year had 12 months of 30 days each plus 5 feast days and an extra day during a leap year.
Note: The event that marks the beginning of an era (a period of time) is called an epoch. For example: the formation of Rome in 753BC is the epoch marking the start of the Roman era. An eon is simply a long period of time, there is no fixed point from which it dates.