The Jewish calendar, which is also referred to as the Hebrew calendar, is used for Jewish religious dates. It is unlike the Gregorian calendar, the calendar used by most of the western world, in several ways. The Jewish calendar starts at the time of creation rather than the birth of Jesus and has 12 months in its regular year, and an extra month, not just an extra day, in its leap year. Days according to the Jewish calendar start at sundown, and the new year occurs in the seventh, not first, month of the year.
Here is a table of Jewish months and their corresponding Gregorian months:
| Number | Name | Length (days) | Gregorian Equivalent |
| 1 | Nissan | 30 | March-April |
| 2 | Iyar | 29 | April-May |
| 3 | Sivan | 30 | May-June |
| 4 | Tammuz | 29 | June-July |
| 5 | Av | 30 | July-August |
| 6 | Elul | 29 | August-September |
| 7 | Tishri | 30 | September-October |
| 8 | Cheshvan | 29 or 30 | October-November |
| 9 | Kislev | 29 or 30 | November-December |
| 10 | Tevet | 29 | December-January |
| 11 | Shevat | 30 | January-February |
| 12 (only in leap years) | Adar | 30 | February-March |
| 12 (13 in leap years) | Adar (Adar II in leap years) | 29 | February-March |
The first thing you might notice is that the Jewish calendar sometimes has an extra month. The years with an extra month are the Jewish calendar’s leap years. Regular years have anywhere from 353 to 355 days, while leap years have 383 to 385 days. Gregorian years have 365 days normally, and 366 days in a leap year. The reason leap years exist is because the standardized days given to the months do not perfectly match up with a solar year—the time it takes for the earth to rotate around the sun. Because the western calendar is based on the solar year, this can be dealt with by adding an extra day to February approximately once every four years.
Because the Jewish calendar is lunisolar, that is, it bases is months on the moon’s rotation around the earth and its years on the earth’s rotation around the sun, an entire month must be added to the year every once in a while. The way this “once in a while” is determined is rather complex.
Unlike the Gregorian calendar where the new year occurs in the first month of the year, the Jewish calendar has Nissan as its first month of the year, and Tishri, the seventh month, as its new year, also called Rosh Hashanah. Therefore, the year increases on this day which usually falls sometime at the end of September or beginning of October according to the Gregorian calendar.
The year according to the Jewish calendar is based on the number of years since creation whereas the year according to the Gregorian calendar is based on the birth of Jesus. Creation was calculated, according to various biblical passages, to be about 3760 BC or BCE. So, for example, sundown on 22 September 2006 was the beginning of the Jewish year 5767 AM. AM stands for anno mundi and means in the year of the world. The days according to the Jewish calendar start at sundown, rather than at midnight as in the Gregorian system.
