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There are three main calendars based on different ways of marking time — the Western calendar, also called the Christian calendar or the Gregorian calendar; the Islamic or Hijri calendar; and the Jewish or Hebrew calendar. Among the difference between these calendars is the core events that they are based on, the lengths of the months and years, and when the day begins.
Perhaps the fundamental way to categorize calendars is by their core event — that is, what the calendar is based on. The Christian calendar is a solar calendar based on Christ’s birth. In religious and traditional secular use, the years are labeled as BC for Before Christ or AD, Anno Domini (in the year of our Lord). More and more, secular uses employ a newer marking system that doesn’t recognize that the dates correspond to Christ’s birth. They mark the year as BCE or CE, where BCE stands for Before the Common Era, and CE stands for the Current Era, Common Era, or Christian Era.
The Gregorian calendar is generally used synonymously with the Western calendar and Christian calendar, but it was actually named after Pope Gregory XIII. It has a year comprised of 12 months and 365 days, 366 in a leap year, which occurs by adding a day in February every four years. While the Gregorian calendar is based on the Julian calendar — a calendar was introduced around 45 BCE by Julius Caesar after consulting an astronomer — it is also based on the year of Christ’s birth. This calendar sought to improve on its predecessors, creating a more regular format than the lunar calendars and Julian calendar. The Gregorian calendar took the place of the Julian calendar around the end of the 16th Century.
The Islamic calendar is based on the emigration of the Prophet Muhammad and his fellow Muslims, the Companions or Sahabah, from Mecca to Medina. The emigration is said to have been commanded by God after many years of Muslim persecution and took place in 622 AD, according to the Western Calendar, or 4382 AM (Anno Mundi, or in the year of the world) according to the Jewish calendar. Hirah is Arabic for emigration, and so the Islamic calendar is also called the Hijri calendar. Years prior to the emigration are labeled as BH, Before Hijra, while years after the emigration are labeled as AH, Anno Hijra or In the year of Hijra. The calendar is based on the lunar year, has about 354 days and 12 months, each with either 29 or 30 days. The names of the months are Muharram, Safar, Rabiul-Awwal, Rabi-uthani, Jumada al-awwal, Jumada al-thani, Rajab, Sha’ban, Ramadan, Shawwal, Dhil-Q’ada, and Dhil-Hijja.
The Jewish calendar is a lunisolar calendar and is based on creation that is said to have occurred — around 3760 BCE according to the Western Calendar. The Jewish or Hebrew calendar has anywhere from 353 to 385 days, and 12 months, 13 in a leap year. Months have 29 or 30 days: Nissan, Iyar, Sivan, Tammuz, Av, Elul, Tishri, Cheshvan, Kislev, Tevet, Shevat, and Adar. In a leap year, Adar I is inserted after Shevat, and the existing month of Adar is called Adar II — the 13th month. While the Western and Islamic calendars have a new year beginning with the first month, the new year according to the Jewish calendar begins in the seventh, not first, month of the year.
Another way these types of calendars differ is when the new day begins. In the Julian, Gregorian, Western and Christian calendars, the day begins at midnight. The Islamic and Jewish calendars, however, begin at sundown.
There are many other types of calendars; the Western calendar is perhaps the most popular, however. Another popular type of calendar — the Chinese calendar — is still used today for Chinese holidays and for astrological purposes. It is a lunisolar calendar with 12 months in a regular year and 13 months every second or third year. Days in the Chinese calendar begin at 11 PM, not midnight. While there is some dispute over when the Chinese calendar began, most believe it began somewhere around 2600 to 2500 BC.