Gemara

(Mishnah): If one sells a Bayis Ir Chomah (house in a walled city), he can redeem it immediately, or any time until 12 months;

The year is counted from the time of his sale - "Ad Melos Lo Shanah";

"Semimah" is extra, to include Chodesh ha'Ibur (the added month in a leap year).

Rebbi says, it includes the extra 11 days in a solar year above a lunar year.

(Beraisa - Rebbi): "Yomim (Tihyeh Ge'ulaso)" forbids redeem his house until at least two days after selling it.

Chachamim expound "Yomim" to teach that the year is counted from day to day.

Rebbi learns this from "Ad Tom Shenas Mimkaro."

Chachamim use that to teach that we count the year from the sale date, not according to the years of the world, which are counted from Rosh Hashanah;

Suggestion: Perhaps he may redeem until the beginning of the day (12 months later)!

Rejection: "Yomim" teaches me'Es l'Es (until the same time of day as the sale).

Rebbi learns from day to day and me'Es l'Es from "Ad Tom Shenas Mimkaro."

31b (Mishnah): At first, one who bought Bayis Ir Chomah would hide on the day 12 months were finished. Hillel enacted that the seller can leave the money in Beis Din, and re-enter his house.

Inference (Rava): If a man said "this is your Get on condition that you give to me 200 Zuz," she is divorced only if he willingly accepts the money from her.

An enactment was needed to allow the seller to return the money against the buyer's will. Normally, this does not work.

18b (Beraisa): All of the following must be full years - the years specified for (certain) Korbanos, the year to redeem Bayis Ir Chomah, the two years before one may redeem a field that he sold, the six years that an Eved Ivri serves, and the years of a boy or girl.

Zevachim 25b (Beraisa): "Ben Shanah" teaches that Korban Pesach must be within one year at the time of Shechitah. "Yihyeh" teaches that all its Avodos must be within its year.

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.

Rishonim

Rambam (Hilchos Shemitah 12:1): If one sells a Bayis Ir Chomah, he can redeem it any time he wants, even on the day of the sale, until 12 months.

Question (Kesef Mishneh and Ri Korkus): Why did the Rambam omit that the 12 months are me'Es l'Es of hours?

Answer #1 (Or Gadol Erchin 9:3): The Rambam holds that me'Es l'Es to the hour applies only to Korbanos. Here, the 12 months end at the end of the day; Me'Es l'Es does not apply to hours. Rather, the Tana'im argue about whether the last day of the year is included in "Ad Tom", i.e. Ad Bichlal.

Answer #2 (Panim Me'iros 2:174): Once the Torah revealed that the year of Bayis Ir Chomah is me'Es l'Es of hours, it is as if it is explicit. He did not need to mention this. The Rambam needed to say so about Kodshim to teach that also Zerikah must be within me'Es l'Es.

Rambam (ibid. 11:9): If one sells a field at a time when Yovel applies, he has no right to redeem it until two years me'Es l'Es from the day of the sale.

Pirush ha'Mishnayos (Erchin 9:3): We are not concerned for hours regarding these laws.

Tosfos (31a DH mi'Yom): R. Elchanan explains that only Bayis Ir Chomah requires me'Es l'Es. For everything else, we can say that (the verse teaches that) we following their year, and not years of the world, so we do not require me'Es l'Es of hours. The Torah revealed this only regarding Bayis Ir Chomah. Tosfos says that all of them require me'Es l'Es. We learn from Bayis Ir Chomah, like the Gemara says below about another matter (when the year ends in Adar).

Rashi (18b DH me'Es): The year (for all of these) ends on the same day and at the same time of the day as they began. We do not follow the years of the world.

Tosfos (Nidah 47b DH Kulan): Zevachim 25b says that hours disqualify Korbanos. The same applies to Bayis Ir Chomah. This is unlike other matters, which do not require me'Es l'Es of hours.

Tosfos (Rosh Hashanah 10a DH Ben): Sometimes, one day is not like a year. Some things require me'Es l'Es, e.g. 13 years and a day for a boy. The day is needed to complete me'Es l'Es.

Divrei Chamudos (Nidah 5:5): To resolve Tosfos in Rosh Hashanah and Nidah, we must say that the second answer is primary. Chachamim always require an extra day to prevent errors, to ensure me'Es l'Es of hours no matter what time of day one was born. Chachamim agree that when we do not need me'Es l'Es, part of a year is like a full year.

Rebuttal (Shach CM 35:1): The years of a boy or girl (and other things) must be me'Es l'Es. Tosfos (47b DH Kulan) does not require me'Es l'Es of hours. Tosfos in Rosh Hashanah also refers to me'Es l'Es of days. However, one answer in Tosfos (44b) requires me'Es l'Es of hours.

Poskim

Beis Yosef (CM 207 DH Matzasi, citing the Agudah): Initially, the buyer would hide on the last day, so the house will be Chalut (permanently his). This shows that if one vowed to do something within a period of time, and there was no Ones until the last day, this is not called Ones. If it were called Ones, Hillel would not have needed to enact!

Magen Avraham (108:11): Forgetting is called Ones. If one forgot to pray, there is Tashlumin (compensation). The Beis Yosef's proof from the Agudah is invalid. There, Ones does not help. It is like one who lent and took a field for collateral, and stipulated that the borrower can redeem in within three years. If Ones occurred and he did not redeem it, the lender keeps it permanently.

Question (Ketzos ha'Choshen 55:1): Rava derived that elsewhere, Nesinah b'Al Korcho is not Nesinah. If a man divorced on condition that she give to him 200 Zuz, and she gave b'Al Korcho, she is not divorced. If one wants to give, but the other refuses to accept, this is Ones. Why did Hillel need to enact? Rather, for Bayis Ir Chomah, if there was an Ones and he could not redeem it the entire year, it does not return to the seller. This is like divorce on condition that she give to him 200 Zuz, and she was unable to give. She is not divorced. We do not say that in a case of Ones, it is as if she did (the stipulation). Here also, the buyer immediately acquires Bayis Ir Chomah, just the Torah stipulated that the seller can redeem for 12 months. If he did not redeem it, even due to Ones, it does not return to him. This is clear from Kesuvos 2b. If one made a Get on condition that he not return within 12 months, and he died, it is not a Get. The Gemara inferred that had he fell sick, it would be a Get, and concluded that Ones does not apply to Gitin. Why don't we say that when he fell sick is different because he could have come earlier, but when he died it is not a Get because divorce does not apply after death! Rather, this shows that Ones at the end of the time is Ones.

Minchas Chinuch (341:7): If Ones occurred and the seller could not redeem, the house is Nichlat. This is why H;illel needed to enact. However, perhaps since the Ones was only on the last day, and he could have redeemed earlier, this is not called Ones. The Agudah derived that this is not called Ones. The Magen Avraham says that normally, Ones on the last day is Ones; Bayis Ir Chomah is different, for it is a sale. (The seller is not 'punished' for Ones; already, it was not his house.) We do not say 'the Torah obligates (the buyer to return the house) in any case of Ones.' Also the Ketzos ha'Choshen says so. This requires investigation. The Gemara says that Bayis Ir Chomah is Ribis, but the Torah permitted it. This shows that it is not a sale. We should say that the Torah exempts Ones (and he does not lose his house)!

Avi Ezri (Ishus 2:21 DH v'Nir'eh): Regarding Bayis Ir Chomah, the six years that an Eved Ivri serves (Hilchos Avadim 2:2), and the years of adulthood, the Rambam did not mention me'Es l'Es. He mentioned it regarding the two years before one may redeem a sold field (Hilchos Shemitah 11:9), and regarding the years of Erchin (Hilchos Erchin 1:4). I say that he holds that since the Beraisa teaches all of them together, all of them are me'Es l'Es of hours. However, at the beginning of the last day, we apply Miktzas ha'Yom k'Kulo (part of the day counts like the whole day). Still, it is not considered the next year until the same hour as the sale. One may redeem Bayis Ir Chomah only within one year, therefore he cannot do so once the day begins (because the year already finished, due to Miktzas ha'Yom k'Kulo). We need a verse to teach me'Es l'Es for Bayis Ir Chomah regarding the Agudah's law. The Agudah connotes that if there was Ones all 12 months, the house is not Chalut. I say that at the beginning of the second year (after me'Es l'Es), we do not say 'the Torah obligates (the buyer to return it) in a case of Ones.' However, if there was Ones until the start of the last day of the year, and then the Ones went away, he can still redeem until the full year (me'Es l'Es). If there was not Ones until now, he could not, for Miktzas ha'Yom k'Kulo. The seller has Zechus to redeem for a year, i.e. until the start of the last day (unless there was Ones). The Rambam omitted hours regarding Eved Ivri, for due to Miktzas ha'Yom k'Kulo, he finishes six years at the start of the day. We do not apply Miktzas ha'Yom k'Kulo to redeeming a sold field after two years (it is the buyer's right to keep it until then), the years of Erchin or of Korbanos.

Aruch ha'Shulchan ha'Asid (Zera'im 34:6): The year to redeem is 12 months from the day he sold, me'Es l'Es, for it says Yomim Tihyeh Ge'ulaso. Even though all numbers in the Torah are not me'Es l'Es, here the Torah requires from hour to hour.