Tosefta zu Joel 2:23

We ask for the rains until Nissan is over, as it says, "And rain will fall for you, the earlier and latter rains during the first [month] (Yoel 2);" [these are] the words of Rabbi Meir. And the Sages say,"Earlier rains [fall in] Marcheshvan, and latter rains [fall on] the first of Nissan (and so these dates are the timetable for prayers for rain). Rabbi Meir said to them, "Knowing that a tree makes fruit for twelve [months], and produce [grows] for six months; what have we found with trees, as it says [about it], "By its month (meaning halfway through the month), it will bear (Yechezkel 47)," so too produce, at its fifteenth day [its season ends]. Haven't you learnt, that earlier and latter rains [fall throughout] Nissan.