Behar, Chapter 1
1) And whence is it derived that the thirty day period before Rosh Hashanah is regarded as the entire year? From "And on the seventh year a Sabbath of resting shall there be for the land" (the month of Rosh Hashanah being regarded as a year in itself) — whence they ruled: The shevi'ith year of benoth shuach (a type of fig) is the second year (after the shevi'ith in which it matured) because it matures every three years. R. Yehudah says: The shevi'ith year of the Persian figs is the end of Shevi'ith, for they mature every two years. They said to him: They stated it only about benoth shuach.