Daf Yomi - Yoma 65 - Definition of a Year ....12 Months or 13?

The Gemora in Chulin proves from פרה אדומה and the שעיר המשתלח that we rely on a majority, for otherwise one could question the validity of these korbanos for perhaps they might be a טריפה. Since the majority of animals are not a טרפה, we are not concerned.

Tosfos asks that perhaps it is not because of רוב, rather it is due to a chazakah? An animal which is a טריפה has a חזקה that it will not live? Tosfos HaRosh comments that this chazakah is applicable by these two קרבנות for they are in their second year and the חזקה states that a tereifa cannot live longer than a year. (See there for explanations as to why this חזקה is not considered a חזקה.)

Reb Shimon Arye Yuzuk asks the obvious question. The שעיר המשתלח is only valid if it is within its first year. How does the Tosfos HaRosh equate this korban with a parah adumah which can be in its second year?

He answers based on our Gemora. The Rosh can hold that regarding the age of an animal in relevance to korbanos, a year is calculated by Beis Din, which would include the extra month by a leap year. However, in regards to an animal living longer than a year when it is a טריפה, he would rule that this is dependent on a twelve month year regardless if there is an extra month. (This is an argument between the שכ and the פרי חדש.) A goat which was born last year in the month of Cheshvan and it was a leap year will still be valid for the korban, for we give it the extra month, yet it would be a proof that it is not a tereifa, for it is in its second year in that regard.

As to what the logic is to make such a distinction, I do not know, but the calculation works!!