Bechoros, Chapter 4, Mishnah 2. We just learned that all the care for a bechor must be done within its first year according to the Jewish calendar. Now we will explore the practical meaning of this rule.
The division of roles between the Yisrael and the kohen:
The Yisrael: Once a bechor is born to him, he does not need to consider at all whether it has a blemish or not - in any case, he must give it to the kohen.
The kohen: If the bechor is unblemished - he must offer it as a korban; and if it has a blemish - he may eat it, provided he does so within that first year. Either way, it is incumbent upon him to offer it or eat it.
If it develops a blemish during its first year:
What is the law if a blemish develops on the bechor in the middle of that year? From now on, it is no longer offered on the altar, but rather it is eaten. And what is the timeframe for eating it? Only the first year, since the original deadline remains in effect. Thus, if ten months have passed since its birth and it was unblemished all that time, and only now it became blemished - only two months remain until the end of the period, during which it must be eaten. One who delays it beyond that violates a positive commandment.
If it develops a blemish after twelve months:
If no blemish developed on it during the first twelve months, and in the meantime it was not offered as a korban either - for example, at a time when the Beis HaMikdash does not exist - and afterward a blemish develops on it, what is the timeframe? Thirty days in which he must slaughter it and eat it, and he may not delay. One who delays also violates a positive commandment. However, even if he did not do so, the obligation still rests upon him to slaughter and eat it, and there is no additional punishment beyond the actual violation of the positive commandment.
In summary: The Yisrael gives the bechor to the kohen in any situation; the kohen offers the unblemished one and eats the blemished one within its first year. If the blemish developed within the year - the time to eat it remains only until the end of that year; if the blemish developed after twelve months - thirty days are given for its slaughter and consumption, and one who delays violates a positive commandment, although there is no additional punishment for it.