We have the Ethiopian calendar. The Bahá'í calendar. The Jewish calendar, the Chinese calendar, the Japanese calendar, the Gregorian calendar, and the Julian calendar, not to be confused with a Julian date.
Ethiopian calendar: The current year according to the Ethiopian calendar is 2003, which began on September 11, 2010 AD of the Gregorian calendar. It has six epagomenal days and so the following year (2004) will begin on September 12, 2011.
Bahá’í calendar: The start of the Bahá'í year is fixed to March 21st in the Gregorian calendar. Leap years thus follow the same pattern as Gregorian leap years. The Bahá'í year 1 began in 1844, marking the start of Babism.
Jewish calendar: Starts with the day when Adam and Eve were created (the Sixth Day of Creation). We are now in the eighth century of the sixth millennium (for example, the year 2010 corresponds to the years 5770-5771).
Chinese Calendar: Feb. 10, 2010 is 4708, The year of the Tiger.
Japanese Calendar: the year 2010 corresponds to Heisei 22. It is the year of the Tiger.
Gregorian Calendar: Dionysius Exiguus
In 525 A.D. a Sythian monk in Rome, Dionysius Exiguus (Dionysius the Little), was preparing new tables for determining the date of Easter and he decided to abandon the pagan calendar in use at the time, that was based on the first year of the reign of Emperor Diocletian (29 Aug., 284). Instead he began a calendar based on his calculated year of the birth of Jesus Christ. Among the biblical data Dionysius had to work with was the following:
Luke 3:1 - Jesus was baptized in the 15th year of the reign of Tiberius
Luke 3:23 says Jesus was about 30 years old at the start of his ministry, His baptism.*
Using this and other data available to him, he calculated the probable year of Jesus birth as occurring in the 753rd since the founding of the Roman empire, which he redesignated the year 1. This method of dating the year was not generally accepted for hundreds of years, but has been nearly universally adopted today.
* It can be demonstrated from the 70 week prophecy of Daniel 9 that the baptism of Jesus occurred in 27 A.D., which would place His birth about or before 3 B.C.