Dating for jewish people
The Jewish calendar, however, coordinates all three of these astronomical phenomena.
Months are either 29 or 30 days, corresponding to the 29½-day lunar cycle.
The Earth revolves around the sun in about 365¼ days, that is, about 12.4 lunar months.
The civil calendar used by most of the world has abandoned any correlation between the moon cycles and the month, arbitrarily setting the length of months to 28, 30 or 31 days.
In addition, Yom Kippur should not fall adjacent to Shabbat, because this would cause difficulties in coordinating the fast with Shabbat, and Hoshanah Rabbah should not fall on Saturday because it would interfere with the holiday's observances. (Before the Common Era), which are commonly used by scholars today.
A day is added to the month of Cheshvan or subtracted from the month of Kislev of the previous year to prevent these things from happening. The "first month" of the Jewish calendar is the month of Nissan, in the spring, when Passover occurs.
For a fascinating (albeit somewhat defensive) article by a nuclear physicist showing how Einstein's Theory of Relativity sheds light on the correspondence between the Torah's age of the universe and the age ascertained by science, see The Age of the Universe. The names are actually Babylonian month names, brought back to Israel by the returning exiles.
Adar I is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the cycle.