Every firstborn that is born in your herd and in your flock, the male, you shall consecrate to Hashem your God; you shall not work with the firstborn of your ox, nor shear the firstborn of your flock.
English: "That is born... you shall consecrate" — from the moment of its birth it is holy; this teaches that you count a year for the firstborn from the moment of its birth.
On the daf / in context: The year within which a firstborn must be eaten is reckoned from the day it is born (with nuances for an unblemished animal that becomes fit only on the eighth day).
Type of Learning
Derash on the verse (Rosh Hashanah) — the year of the firstborn begins at birth.
What Is Learned
The year allotted for the consumption of a firstborn is counted from the day of its birth.