Sifrei Bamidbar 64

(Bamidbar 9:1) "And the L-rd spoke to Moses in the desert of Sinai in the second year after their going out from the land of Egypt, in the first month (Nissan), saying": This verse is in praise of Israel (to indicate) that they were encamped before Mount Sinai for eleven months, and to teach that there is no "before and after" (i.e., chronological order) in the Torah. For in the beginning of this Book it is written (1:1) "And the L-rd said to Moses in the desert of Sinai in the tent of meeting on the first day of the second month (Iyyar), and here it is written "in the first month" — to teach that there is no "before and after" in the Torah. Rebbi says: This is not needed (for this teaching), for it is already written (Shemot 16:35) "And the children of Israel ate the manna for forty years until they came to an inhabited land" — and they had not yet done so.

And to teach that we count (the years) from the exodus from Egypt, it being written "in the second year after their going out from the land of Egypt." When they came to the land, they began to count (shemitah, etc.) from their coming, as it is written (Vayikra 25:2) "When you come to the land, etc." When the Temple was built, they began to count from its building, viz. (I Kings 9:10) "And it was at the end of twenty years from Solomon's building, etc." When the Temple was destroyed, they began to count from its destruction, viz. (Ezekiel 40:1) "… in the fourteenth year of the city's being smitten." When the captivity intensified, they began to count from the captivity, viz. (Daniel 2:1) "And in the second year of the reign of Nevuchadnezzar, etc.", and (Chaggai 1:1) "And in the second year of King Darius, etc." And just as they counted for years, so they counted for months (e.g., [Shemot 19:1]) "In the third month of the exodus of the children of Israel from the land of Egypt."