וַיָּבֹ֣אוּ בְנֵֽי־יִ֠שְׂרָאֵל כָּל־הָ֨עֵדָ֤ה מִדְבַּר־צִן֙ בַּחֹ֣דֶשׁ הָֽרִאשׁ֔וֹן וַיֵּ֥שֶׁב הָעָ֖ם בְּקָדֵ֑שׁ וַתָּ֤מָת שָׁם֙ מִרְיָ֔ם וַתִּקָּבֵ֖ר שָֽׁם׃
И вошли сыны Израилевы, и все общество, в пустыню Зин в первый месяц; и люди живут в Кадесе; и там умерла Мириам и была похоронена там.
Tosefta Rosh Hashanah
How so, "for months"? Even though it says, "on the first month" and "on the second month"*i.e., in various verses in the Torah (see, e.g., Num. 20:1 ("in the first month"), Gen. 8:14 ("in the second month")), in which the ordinals "first" and "second" do not refer to a specific month of the year, but rather indicate the first or second month after a specific event (see Lieberman), they only begin to [formally] count from Nisan, as it says (Ex. 12:2), "This month shall be for you the first (lit., 'head') of the months."