TT #45 · Parshas Tazria · Aliyah 1 · Passuk 6
English: On the completion of her period of purification, for either son or daughter, she shall bring to the priest, at the entrance of the Tent of Meeting, a lamb in its first year for a burnt offering, and a pigeon or a turtledove for a purgation offering.
Chabad.org — Chumash, English translation + Rashi for Vayikra 12:6
Torah Temimah cites Arakhin 18b for the rule that an animal’s year for consecrated offerings is reckoned from its own birth, not by the calendar year.
E-Daf — Arakhin 18b · Shas.org · Sefaria — Arakhin 18b, source text and translation
English: The year stated for consecrated offerings is measured from the offering's own birth date: “its year” means its own year, not a calendar year.
Textual definition
For an offering that must be brought during the animal’s first year, the year is measured from the animal’s birth, rather than ending at the close of a calendar year.