What is our timetable?

The Gregorian calendar  

System-defined rules to designate the divisions of time in years, months, weeks, days, dates and holidays. It is assumed that the first rudiments on the subject date back to 2000 a. c., when alincamientos of stones were built to determine the passage of each day, the time of repetition of the Moon or reiteration of the stations. The oldest known calendars were based on the Moon and its phases, because it provided certain observers consistent repeatable and predictable then.
The India, China, Mesopotamia, Greece, Rome, they had different types of calendars and the Maya divided the year into months of 18 to 20 days with a five-day period at the end of the year for ceremonies or rituals.
The Aztec calendar stone is a disc carved with exquisite fastness and precision, headed to the Center by the image of the Sun with the tongue out, indicating that it is shining. His face is surrounded by the four cardinal points or the four creations of the world and their consequent disasters.
The first destruction produced by the Tiger; the second, by the winds; the third, by the rains and the fourth, by the flood. They lived the fifth time or the Fifth Sun.
As the strict solar year has a duration of 365 days, 5 hours, 48 minutes and 46 seconds, they accumulated those hours more and produced serious discrepancies with the passage of time.
In times of Julio Cesar, the Romans managed a calendar with sufficient errors as to be called to the astronomer Sosígenes to frame it. This is how the year 46 BC. be awarded 445 days.
In 1582, Pope Gregorio XIII ordered that they saltearan 10 days and leap years only were it when they are divisible by 400. They are determined, in addition, when the last two digits are divisible by 4 (1912, 1916, 1944, 1996). The secular years are not leap unless their first two digits are divisible by 4: 1900 was not leap to 2000 Yes.
From the Gregorian calendar, all years have 365 days and every four years would be a leap of 366. Leap comes from the latin because according to the Roman reckoning on February 28 that in ordinary years was the sixth kalendas martzz, with the addition of one day to February it became a bl' sixth kalen das martii.
The year 2000 on the Gregorian calendar corresponds to the 6236 old Egyptian calendar; to the 5760 Jewish calendar; to the 5119 the ancient maya calendar and the 1420 of the Muslim calendar.
The Platonic year, perfect year or great year, Annus magnus, is a cycle that the Greek astronomers estimated at 26,000 years, at the end of which all celestial bodies would returned to the same place they occupied at the time of the creation.
Article translated for educational purposes from: Planeta Sedna