In Talmudic times the regulation of the calendar was the exclusive right of the Jewish leadership in the Land of Israel, particularly that of the Nasi (Patriach).
Since that time, such regulation has been regarded as a task of crucial importance for the observance of Judaism. each month begins with the "birth" of the new moon).
The first 2,000 years from the creation of Adam to the time of Abraham (Avraham) is known as the period/age of desolation (Tohu).
The next two thousand years from the time of Abraham to the expected arrival of the Jewish Messiah (Mashiach) is known as the period/age of Torah. ---Olam Habah | | | | | Days of: Tohu Torah Messiah Athid Lavo (------------- Olam Hazeh ----------------) The 7,000-year plan of the God of Israel is a major concept and foundational truth in understanding Bible prophecy and eschatology (study of the last things).
"The main purpose of the Jewish calendar is, and always has been, to set the dates of the festivals.
The final 2,000 years of time within the 6,000 years of the present/age world (Olam Hazeh) is known as the days of the Messiah (Yemot Mashiach). The rabbis of traditional Judaism understand that the God of Israel gave the seven days of creation in the book of Genesis as Torah/instruction to teach His people that there are 7,000 years of time from the creation of Adam and Eve in the Garden of Eden (Gan Eden) to the end of the Messianic Age (Athid Lavo).