Rashi: It is speaking to the Kohen, since we already know that a B'chor is one of the Matnos Kehunah - irrespective of whether it is a Tam or a Ba'al-Mum. ¹

Rashi #1: It implies that one may not hold back a B'chor from the Kohen until the year after it is born. ¹

Rashi #2: It implies that one is permitted to eat a B'chor from one year to the next. ² Assuming it is offered on the last day of its year, it may be eaten on the first day of the following year - which teaches us that a B'chor may be eaten for two days and the night in between ? like a Shelamim.

Bechoros, 26b #1: It implies that a B'chor must be eaten within its first year - whether it is a Tam in the time of the Beis-Hamikdash or a Ba'al-Mum nowadays. ³

Bechoros, 26b #2: It implies the first year of the B'chor from the eighth day after its birth ⁴ until the same date one year later - which encroaches on the years of the world. ⁵

Arachin, 31b: We learn via a Gezeirah Shavah ""Shanah" "Shanah" from Batei Arei Chomah ⁶ that, if two firstborn lambs are born in Reuven's flock, one on the fifteenth of Adar Rishon and one on Rosh Chodesh Adar Sheini, the year of the second lamb terminates on Rosh Chodesh Adar of the following year, and of the first one on the fifteenth.