(Beraisa): "Seh Samim Zachar Ben Shanah Yihyeh" - a Pesach must be Tam (unblemished) and (at most) one year old at the time of Shechitah.

"Yihyeh" teaches that all its Avodos must be when it is one year old.

Question: If it was one year old at the time of Shechitah, how could it be two years old at the time of Zerikah?

Answer (Rava): This teaches that we calculate the age of Korbanos from hour to hour.

26a - Question (Abaye - Beraisa - R. Yehoshua): If a k'Zayis of the meat remains, we may throw the blood (even though this is the ultimate blemish)!

Answer: Rather, "Yihyeh" teaches only that it must be one year old at the time of all its Avodos (but it may be blemished).

Erchin 18b (Beraisa): All of the following are full years me'Es l'Es - the years for Korbanos, the year for Bayis Ir Chomah, the two years before redeeming a field, the six years of an Eved Ivri, and the years of a boy or girl.

Question: What is the source of this regarding Korbanos?

Answer (Rav Acha bar Yakov): "Keves Ben Shenaso" - it must be in its first year (it need not be in the same calendar year, from Tishrei to Tishrei).

Bechoros 27b: The Mishnah says "if a Bechor developed a Mum within its year..." This implies that we count a year from its birth.

(Rav): We learn from "Lifnei Hash-m Elokecha Sochalenu Shanah v'Shanah." The year from its birth enters the next calendar year.

(Rav Acha bar Yakov): Bei Rav learns Bechor from other Kodshim, and other Kodshim from "Keves ben Shenaso."

Rosh Hashanah 6b (Beraisa): Regarding Bechor and all Kodshim, Bal Te'acher applies after a year without (three) Regalim.

According to the opinion that the Regalim need not be in order, Rebbi can say that animal was Hukdsh after Pesach, and it was a leap year. Rabanan say that a leap year is 13 months (for Bal Te'acher). They must say that this year Sivan 6 was (a day) after Shavuos, and next year it was before Shavuos.

(Abaye): We count the year of Bechor from the day of birth.

(Rav Acha bar Yakov): We count from when it could be offered.

They do not argue. Abaye discusses a Bechor born with a Mum, and Rav Acha discusses a Tam (an animal without a Mum).

Rambam (Hilchos Ma'aseh ha'Korbanos 1:11): Small animals (e.g. Kevasim Benei Shanah) are from eight days until a full year from day to day. If it was a leap year, the extra month is included in the year. We offer cattle until three full years from day to day, and Tzon (sheep and goats) until two full years. More than this is old, we do not offer it.

Avi Ezri (Ishus 2:21 DH v'Nir'eh): The Rambam holds that everything in the Beraisa (Erchin 18b) has the same law. Letter of the law, the year extends me'Es l'Es of hours. However, part of the day is like the entire day, therefore it is as if the year finished from the start of the day, but the new year does not start until me'Es l'Es of hours.

Rambam (13): We count hours regarding Kodshim. If it is an hour too young or too old it is Pasul. Even if a Korban (that must be Ben Shanah) was (within) one year old at the time of Shechitah but more than a year at the time of Zerikah, it is Pasul.

Rambam (Hilchos Bechoros 1:8): A Bechor must be eaten within its year, whether it is Tam or Ba'al Mum - "Lifnei Hash-m Elokecha Sochalenu Shanah v'Shanah." If it is Tam we count the year from day eight, when it was fitting to be offered. If it was born with a Mum, and Kalu Chadashav (it was born after a full term pregnancy), we count the year from its birth, for it was permitted immediately. If we do not know that Kalu Chadashav, we count the year from day eight.

Rambam (12): The year for a Bechor is a full lunar year of 12 months from day to day. If it was a leap year, it is 13 months.

Question: In Erchin we expound "Shenaso" (its year) to teach that we do not follow the calendar year. We could deduce this without the verse! Temidim and (most) Musafim of Rosh Hashanah are Kevasim Benei Shanah. If we increment an animal's age every Rosh Hashanah, any animal is at least two years old on Rosh Hashanah, unless it was born that day. A Korban must be at least eight days old!

Answer #1 (cited in Tosfos Erchin 18b DH Shenaso): We count an animal's year starting from when it may be offered, i.e. from the eighth day, just like regarding Bechor (Rosh Hashanah 6b). Without "Shenaso" we would say that the "Kevasim Benei Shanah" of Rosh Hashanah must be exactly eight days old.

Rejection (Tosfos ibid.): We count from the day it may be offered regarding 'Bal Te'acher' (delaying bringing Korbanos), but not regarding its age. Zevachim 25b says that an animal can be one year old at the time of Shechitah, and two years old at the time of Zerikah, because we calculate the age of Korbanos from hour to hour. If the age is counted from when it may be offered, i.e. dawn of its eighth day, Shechitah and Zerikah cannot be in different years, for both must be the same day!

Answer #2 (Tosfos ibid.): Without "Shenaso" we would not know that we calculate the age of Korbanos from hour to hour (just like regarding Bayis Ir Chomah - Erchin 31b).

Turei Even (Rosh Hashanah 9b DH Pachos): In Bechoros, we learn from "Shanah Shanah" that Bechor is eaten in its year. Normally, when we count calendar years, Tishrei begins the next year only if at least 30 days passed. If this applied to Bechor, even if we counted calendar years, one year could enter the next (if it was born less than 30 days before Rosh Hashanah)! However, if we count its years, this occurs more frequently, so presumably this is the Torah's intent. However, why do we need Shanah Shanah to teach that we follow its year? If we would not say so, hours could not disqualify Kodshim! We must say that without Shanah Shanah, we would have expounded "Yihyeh" to teach that the Korban must be Tam until Zerikah, and not that it must be a yearling until Zerikah.

Aruch ha'Shulchan ha'Asid (Kodshim 63:26): Tosfos' proof is not a proof. However, Rashi (Shabbos 136 DH Havah Achlinan) also does not require me'Es l'Es. It appears that we count hours only at the end of the year, but not at the beginning, for then part of a day is like the entire day.

Shulchan Aruch (YD 15:2): If we know that an animal had a full gestation, i.e. nine months for a big (work) animal or five months for a small animal, it is permitted the day it was born. If we are unsure, it is a Safek Nefel until the eighth night begins.

R. Akiva Eiger: The Taz (2, b'Sof) connotes that we need seven 24-hour periods. We do not say that part of the day of birth is like the entire day. The Pri Megadim agrees. Tevu'os Shor is lenient. I agree. R. Shimon ben Gamliel supported himself from the verse "Shiv'as Yamim Yihyeh Im Imo." Tosfos (Erchin 18b DH mi'Shenaso) proves that we do not need seven 24-hour periods. They proves that the year to offer a Korban (that must be offered within the year) is from the day of birth, and not from the day when it could be offered, since we say that hours disqualify Kodshim. I.e. it is possible to enter its second year between Shechitah and Zerikah. If we count from when it could be offered, this law could not apply! (It would enter its second year exactly one year from the eighth day, at the earliest time of the day to offer a Korban. This cannot be between Shechitah and Zerikah.) If we required seven 24-hour periods, the second year could begin in the middle of the day (at the time it was born), which could be between Shechitah and Zerikah.

Noda bi'Yehudah (2:YD 7, from his son) brings R. Akiva Eiger's proof, but concludes that if a case occurred, he would not be lenient against great Poskim (the Taz and Pri Megadim). However, one who is lenient does not lose.

Shulchan Aruch (306:7): A Bechor, even a Ba'al Mum, must be eaten within its year.

Taz (6): This is so one will not transgress shearing it or working with it (these are forbidden even if it has a Mum).

R. Akiva Eiger (DH b'Gizah): This is like the Yere'im and Levush, who say that the verse ("Lifnei Hash-m Elokecha Sochalenu Shanah v'Shanah") applies to a Tam, but the Isur regarding a Ba'al Mum is mid'Rabanan. The Rambam and Semag explicitly say that it is mid'Oraisa even regarding a Ba'al Mum.

Rashash (Bechoros 28a DH Aval): Teshuvas Shivas Tziyon (24) proves that the Mitzvah to eat a Ba'al Mum within the year is mid'Rabanan. We said (Bechoros 28a) that we always allow 30 days from when the Mum came, even if this extends past the year, due to Hashavas Aveidah. This is not a proof. The Rambam holds that mid'Oraisa one has a year from when the Mum came, just like one has a year for a Tam from when it could be offered. Mid'Rabanan, one has only until a year from birth; because of Hashavas Aveidah, one always gets at least 30 days.

Turei Even (6b DH Mishkachas): Rebbi holds that 365 days after Pesach (in a leap year) is a year without Regalim. Tosfos says that Rabanan hold that if one was Makdish during (e.g.) Sukos, the start of Sukos next year joins with the end of this Sukos to comprise one Regel. I infer that the years of Bal Te'acher are from day to day and from hour to hour. If one vowed during the Regel at six hours of the day, he transgresses Bal Te'acher at six hours of that day the next year. If it did not depend on the hour, he would transgress once the previous night begins, and we could find a year without Regalim if one vowed during a Regel during the night or day. The year ends when that night begins the next year, but three Regalim are not completed until the same time of the day. Surely, for Regalim we require from hour to hour. The Gemara says that Regalim do not begin from night. If one vowed during the first night of Yom Tov, when the Regel finishes the first Regel was not yet finished, like Tosfos said. What is the source to require from hour to hour for Bal Te'acher? Often, years of the Torah are from day to day, but not from hour to hour! We can say that we learn from a Ribuy regarding Bechor (the Sifri learns from "Kol ha'Bechor"). We say below that the year of Bechor begins from when it was ready to be offered, i.e. the beginning of the eighth night. Its year is always from hour to hour, since it always begins at the beginning of the night, even if it was born during the day. Likewise, we require from hour to hour for all Kodshim, even though they could have a year not from hour to hour, e.g. he vowed in the middle of the day. This is unlike judging what is possible from what is impossible, for the Torah revealed about Bal Te'acher regarding Bechor, so surely Bal Te'acher for all Korbanos must resemble Bechor. Do not say that the Gemara could have said that a year without Regalim applies only to Bechor, for it says that Bechor and all Kodshim are the same.

Shulchan Aruch (8): If the Mum came during the year, one may keep it all 12 months.

Shach (14): The Tur says that when the Mum came later, one may keep it 12 months from its birth. The Beis Yosef explains that this is if Kalu Chadashav. Otherwise, we count the year from day eight. This is unlike Rashi and Tosfos, who explicitly say that we count from birth only if Kalu Chadashav and it was born with a Mum. It seems that the Rambam and Semag agree. The Gemara supports them, for it asked why we count a Ba'al Mum's year from its birth, before it may be eaten. To answer this we must say that Kalu Chadashav and it was born with a Mum! It seems that our text of the Tur is mistaken. It should say 'one may keep it 12 months from the eighth day from its birth.' The previous words of the Tur prove that he holds like this; Kalu Chadashav does not help, since it was born Tam and could not be eaten immediately anyway.