In the Gregorian calendar, a year is a leap year if the year number is evenly divisible by 4. The number must be divisible by both 100 and 400 to be a leap year; otherwise it is not a leap year.
For example, 16 are leap years, but 1700, 1800, and 1900 are not.
The instant, date, or year from which time is marked is called the epoch of the era.
There are many different calendar eras such as Saka Era.
So the first year of the 21st century begins in 2001 AD and ends with the beginning of 2002 AD).
It took about 400 years for the dating system devised by Dionysius to reach common usage.
In antiquity, regnal years were counted from the accession of a monarch.
Astronomers have identified this eclipse as one that took place on 15 June, 763 BC, which has allowed absolute dates of 892 to 648 BC to be assigned to that sequence of eponyms.However, its adoption in the United Kingdom and other countries was fraught with confusion, controversy, and even violence.It also had a deeper cultural impact through the disruption of traditional festivals and calendrical practices." ( The widespread use of the Gregorian calendar and the use of BC and AD throughout the world came about thanks to the colonization practices of Europe.Each year at the Akitu festival (celebrating the Mesopotamian new year), one of a small group of high officials (including the king in later periods) would be chosen by lot to serve as the limmu for the year, which meant that he would preside over the Akitu festival and the year would bear his name.The earliest attested limmu eponyms are from the Assyrian trading colony at Karum Kanesh in Anatolia, dating to the very beginning of the 2nd millennium BC, and they continued in use until the end of the Neo-Assyrian Period, ca. Assyrian scribes compiled limmu lists, including an unbroken sequence of almost 250 eponyms from the early 1st millennium BC.