Although Rosh HaShanah marks the change of the Jewish calendar year, Nisan is considered the first month of the Hebrew calendar. The Mishnah indicates that the year of the reign of Jewish kings was counted from Nisan in Biblical times. Nisan is also considered the beginning of the calendar year in terms of the order of the holidays.
In addition to this New Year, the Mishnah sets up three other legal New Years:
(Adapted from Wikipedia)