As to the beginning of the 70 weeks, Nehemiah was granted permission by King Artaxerxes of Persia, in the 20th year of his rule, in the month of Nisan, to rebuild the wall and the city of Jerusalem. (Ne 2:1, 5, 7, 8)
... In his calculations as to the reign of Artaxerxes, Nehemiah apparently used a calendar year that began with the month Tishri (September-October), as does the Jews’ present civil calendar, and ended with the month Elul (August-September) as the 12th month.