Malbim Ayelet HaShachar Klal 310
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יש הבדל בין שנה סתם שחשבינן תמיד לשנות עולם ובין כשאמר בן שנתו שחשבינן לשנה שלו מיום אל יום. (תזריע ס' כד)
Ayelet Hashachar, translated by Betzalel Avraham Feinstein, 2021
There is a difference between ordinary שנה (year), which is always calculated with respect to the calendar year starting with Rosh Hashanah, and בן שנתו (one year-old), in which the year begins on one date and ends on the same date of the following year.