Jewish Calendar – Hebrew Calendar

On October 7, 3761 BC, the epoch (origin) of the modern Hebrew calendar. From the article:

"Sun, Moon, and Holy Scripture
Jewish time reckoning is lunisolar, which means that the calendar keeps in sync with the natural cycles of both the Sun and the Moon. Featuring a body of complex regulations, exceptions, and mathematical rules, it is also designed to satisfy a number of requirements conveyed in the Jewish Holy Scripture.

The Hebrew calendar is a comparatively imprecise system in terms of reflecting the duration of a solar year, which is the time it takes Earth to complete a full orbit around the Sun. In comparison with the timing of the astronomical seasons, it is off by 1 day every 216 years.

Calendar Structure
Months in the Jewish Calendar
Month Names Number of Days
Nisan 30
Iyar 29
Sivan 30
Tammuz 29
Av 30
Elul 29
Tishrei 30
Marcheshvan (Cheshvan) 29 or 30
Kislev 30 or 29
Tevet 29
Shevat 30
Adar 29
A year in the Hebrew calendar can be 353, 354, 355, 383, 384, or 385 days long.

Regular common years have 12 months with a total of 354 days. Leap years have 13 months and are 384 days long. Months with uneven numbers usually have 30 days, while months with even numbers have 29 days.

In addition to these regular (kesidrah) year lengths, both common and leap years can be a day shorter (cheserah or deficient year with 353/383 days) or a day longer (shlemah or complete year with 355/385 days).

These alterations are designed to prevent Rosh Hashana and other holidays from falling on certain days of the week. In practice, a day is added to the 8th month (Marcheshvan) or subtracted from the 9th month (Kislev).

In civil contexts, a new year in the Jewish calendar begins on Rosh Hashana on Tishrei 1. However, for religious purposes, the year begins on Nisan 1.

When Was Year 1?
According to Hebrew time reckoning we are now in the 6th millennium. The Hebrew year count starts in year 3761 BCE, which the 12th-century Jewish philosopher Maimonides established as the biblical date of Creation.

Years in the Jewish calendar are designated AM to identify them as part of the Anno Mundi epoch, indicating the age of the world according to the Bible. For example, the beginning of the year 2020 in the Gregorian calendar converts to year AM 5780 in the Jewish calendar.

Leap Year Rules
Like in the Islamic calendar, months in the Jewish calendar are based on the phases of the Moon. Each month begins with the appearance of a Crescent Moon after the New Moon phase and lasts for a full lunation, a Moon cycle encompassing all phases of the Moon.

Since the sum of 12 lunar months is about 11 days shorter than a solar year, a leap month is added every 2 to 3 years, or 7 times in a 19-year cycle. Leap months are meant to keep the calendar in step with the astronomical seasons and make sure that the religious observances occur at the correct time of year, as mentioned in the Torah.

History and Background
The Jewish calendar is based on a history of time reckoning efforts dating back to ancient times. Both Israelite and Babylonian influences played an important role in its development. According to the account of Persian astronomer al-Khwarizmi (c. 780 – 850 CE), most of the features of its modern-day version were in place by the 9th century CE.

In parallel with the modern Islamic calendar, the timing of the months in the early forms of the Jewish calendar depended on actual sightings of the Crescent Moon. However, this practice was gradually changed, and by 1178 CE the calculation of the beginning of a new calendar month had been fully replaced by the mathematical approximation of the moment the Crescent Moon begins to appear (Molad) rather than actual sightings."