English: "Year by year" — the year of the firstborn is its own year. From where is this derived? From the verse "before Hashem your God you shall eat it year by year"—which is a year that enters into its fellow? It must be the year of the firstborn.
On the daf / in context: The year is personal to the animal, counted from its own birth date rather than from a fixed calendar date.
Type of Learning
Derash on the verse (Bekhoros) — the year is counted from the animal’s own birth.
What Is Learned
The year within which a firstborn must be eaten is counted from the day of that animal’s birth.