Since before recorded history humans have been interested in keeping track of time and have invented many differant ways to record it. One of the earliest methods we know of is the Mayan Calendar. The Mayan calendar is not a single calendar, but rather a series of calendars. They are the Mayan Long Count and the Mayan Calendar Round. The Mayan calendar round is itself a combination of two calendars, namely the Tzolkin calendar and the Haab calendar.
A Mayan Long-Count date is specified by a sequence of five fields. The names of the fields are baktun, katun, tun, winal, and kin.
1 kin is 1 day
1 winal is 20 kin, which is 20 days (a long-count month)
1 tun is 18 winal, which is 360 days (a long-count year)
1 katun is 20 tun, which is 20 long-count years
1 baktun = 20 katun, which is 400 long-count years
The first day in the Long-Count calendar is 0 baktun, 0 katun, 0 tun, 0 winal, 0 kin. It is written as 0.0.0.0.0 and it corresponds to 11 August 3113 B.C. in the Gregorian calendar.
Speaking of the Gregorian calendar, it is internationally the most widely accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named. The reformed calendar was adopted later that year by a handful of countries, with other countries adopting it over the following centuries.
The motivation for the Gregorian reform was that the Julian calendar assumes that the time between vernal equinoxes is 365.25 days, when in fact it is presently almost 11 minutes shorter. The discrepancy results in a drift of about three days every 400 years. At the time of Gregory's reform there had already been a drift of 10 days since Roman times, resulting in the spring equinox falling on 11 March instead of the ecclesiastically fixed date of 21 March, and moving steadily earlier in the Julian calendar. Because the spring equinox was tied to the celebration of Easter, the Roman Catholic Church considered this steady movement in the date of the equinox undesirable.
Moving on to the computer age, Unix time is a system for describing instances in time, defined as the number of seconds that have elapsed since midnight Coordinated Universal Time (UTC), 1 January 1970, not counting leap seconds. It is used widely in Unix-like and many other operating systems and file formats. The Unix time number is zero at the Unix epoch, and increases by exactly 86 400 per day since the epoch.
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