How old is the jewish calendar?

Answer 1

We are now in the year 5772 in the Jewish lunisolar calendar. The existence of twelve lunar months plus an intercalated thirteenth month every 2-3 years, is as old as the Torah.

Today, the entire calendar is of a fixed cycle. In the First Temple times and earlier, the setting of the New Month (Rosh Chodesh) was declared each month by the Sanhedrin (High Court of sages) after they heard testimony of witnesses who had seen the new moon. There were some limiting rules, such as every month having either 29 or 30 days, and not letting Rosh Chodesh slip more than two days before or after the calculated appearance of the new moon (such as in the winter, when clouds hide it); but otherwise the sages had a degree of prerogative (Talmud, Rosh Hashanah 21 a). In Second Temple times, because there were already far-flung Jewish communities (Rome, Jerba, Tripoli, Persia), the sages limited their flexibility especially with regard to the month of Elul always having 29 days (ibid, Rosh Hashanah 19b), so that the holidays of Tishrei should be celebrated everywhere on the same dates.

Around 1650 years ago, Roman persecution made it impossible for the Sanhedrin to continue congregating in a regular fashion, so the sages decreed and publicized further details to the calendar which would henceforth be inflexible. At that time the calendar became what we have today (see the Jewish law code Tur Orach Chaim, ch. 428). The final cementing of the last details took place in the time of the Gaonim (Sages of the yeshivoth in Babylonia) some 12 centuries ago.

Answer 2

September 25, 2014 was the first day of Rosh Hashanah, and began the year 5775 of the Jewish ritual calendar. That date counts from the first verses of Genesis, which describe God's creation not only of the earth, but of all of space-time.

Answer 3

as of 2011, it is 5,772 years, but this is only a traditional number. Most Jews recognize that the Earth is more than 4.6 billion years old.