The Jewish calendar, like the Chinese calendar and Islamic calendar, is a lunisolar calendar. This means that the Jewish calendar is a combination of a solar calendar and a lunar calendar. The Jewish calendar attempts to have the year coincide with the solar calendar and its months coincide with the lunar calendar. Since the twelve months of the Jewish calendar are about eleven days short of a 365-day year, a leap month is added to the calendar on a 19-year cycle developed by Hillel II in the fourth century. The current cycle began in 5758 (1997-1998). An extra month is added on the third, sixth, eighth, eleventh, fourteenth, seventeenth, and nineteenth years during this cycle. (This is why one will see Adar I and Adar II on some calendars).

The year is calculated as the number of years since creation (5772 = September 2011 - September 2012) The year was calculated by adding up the ages of people in the Tanach back to the time of creation. However, this does not necessarily mean that the universe has existed for only 5770 years as we understand years. An ordinary Jewish calendar year has between 353 and 355 days and a leap year has between 383 and 385 days. Each month begins on Rosh Chodesh (new moon).

5772 (2012) Omer Calendar