Gemara

13a (Rav Nachman citing Rav): The Halachah follows R. Yehudah.

28b (Beraisa - R. Yehudah): One who eats Chametz before the time or after the time transgresses a Lav. During the time he transgresses a Lav with Kares;

R. Shimon says, before the time or after the time he does not transgress. During the time he transgresses a Lav with Kares;

From the time one may not eat it, it is Asur b'Hana'ah.

This is according to R. Yehudah.

(R. Yehudah): "Lo Sochal Alav Chametz" forbids eating Chametz after six hours [on the 14th].

Objection (R. Shimon): "Lo Sochal Alav Chametz Shiv'as Yomim Tochal Alav Matzos" teaches that the Lav to eat Chametz and the Aseh to eat Matzos apply at the same time (i.e. starting at night).

R. Yehudah uses "v'Lo Ye'achel Chametz," "Kol Machmetzes Lo Sochelu," and "Lo Sochal Alav Chametz" to forbid before the time, during the time, and after the time;

R. Shimon applies the last verse during the time. The first two teach about the Isur of Chametz at the time of Yetzi'as Miztrayim, and Chametz leavened without Se'or.

29a - Question: The Tana of our Mishnah forbids a Yisrael's Chametz after Pesach. This is unlike R. Yehudah. He forbids [even a Nochri's] Chametz after Pesach! It is not R. Shimon. He permits even a Yisrael's Chametz [after Pesach]!

Answer (Rava): Really, it is R. Shimon. R. Shimon fines [to forbid Chametz of a Yisrael after Pesach] because he transgressed Bal Yera'eh and Bal Yimatzei.

30a (Rava): After Pesach, Rav Nachman would tell us to buy Chametz of Nochrim [that was made during Pesach. This is like R. Shimon.]

21a (Mishnah): As long as it is permitted to eat [Chametz on Erev Pesach], one may feed it to animals, or sell it to a Nochri. One may benefit from it;

After the [permitted] time expires, it is Asur b'Hana'ah.

(Mishnah - R. Meir): One may eat Chametz until the end of the fifth hour. We burn it at the beginning of the sixth hour;

R. Yehudah says, one may eat until the end of four hours. We are Toleh (do not eat, but do not burn) during the fifth hour, and burn at the beginning of the sixth hour.

Rishonim

Rambam (Hilchos Chametz 1:8): One may not eat Chametz from midday of the 14th of Nisan. One who eats then is lashed mid'Oraisa. "Lo Sochal Alav Chametz" forbids from the time proper to slaughter Korban Pesach, i.e. after midday.

Rebuttal (Ra'avad): We hold like R. Shimon, regarding before and after the time. He does not expound these verses to discuss before and after the time. He learns an Isur [after midday] from "ba'Yom ha'Rishon Tashbisu Se'or", or from "Lo Sishchat Al Chametz" like Rava (5a), but there is no Torah Isur Hana'ah or lashes.

Magid Mishneh: R. Yehudah says that a Lav forbids Chametz before and after the time. The Halachah follows R. Shimon after the time, like a Stam Mishnah. For before the time, we rely on the general rule that the Halachah follows R. Yehudah against R. Shimon. The arguments before and after the time are based on different verses. They do not depend on each other. Rava gives a source to forbid Chametz after midday on Erev Pesach (5a), even though he rules like R. Shimon [after the time]. The Ge'onim and the Ritz Gei'us rule like this. Hilchos Pesukos forbids b'Hana'ah in those hours. It seems that he forbids mid'Oraisa. The Ra'avad and others hold that there is only an Isur Aseh before night. The Rambam and Ge'onim are primary.

Rosh (2:4): The Ra'avad, Ba'al ha'Ma'or and Ba'al ha'Itur rule like R. Shimon. However, Ba'al ha'Itur says that R. Shimon forbids Hana'ah mid'Rabanan, and Ba'al ha'Ma'or says that he permits eating it until evening, and eating fulfills Bi'ur. The Rambam and Ritz Gei'us say that one is lashed [for eating it then]. I agree. The Halachah follows R. Shimon after the time, but before the time we hold like R. Yehudah, like the Stam Mishnah (21a). Rebbi acted on this in practice, and commanded to sell a deposit of Chametz in the fifth hour (13a).

Gra (OC 443:1 DH Chametz): If Rebbi permitted after the fifth hour, he would have waited longer! We establish him to hold like the Tana who says that even if a deposit is perishing, we leave (do not sell) it.

Rosh (ibid.): We establish our Mishnah like R. Shimon regarding after the time. Rava did not say "the Halachah follows R. Shimon", which would connote totally, rather, "Chametz after the time is permitted, like R. Shimon." We hold like R. Shimon's Drashah "Machmetzes" to teach about what became Chametz by itself, but we do not expound "Lo Ye'achel" like R. Shimon.

Poskim

Shulchan Aruch (OC 443:1): Chametz from six hours and onwards on Erev Pesach is Asur b'Hana'ah.

Beis Yosef (DH Chametz): Rashi explains that 'before the time' is from six hours until the night. The custom of Yisrael is to forbid Hana'ah after six hours. One who rules leniently like the Ba'al ha'Ma'or, who is a lone opinion, against the custom and the Rabim, should be censured.

Magen Avraham (1): Bal Yera'eh does not apply until night. There is only an Aseh of Tashbisu.

Gra (DH Chametz): The Rosh brings a proof from Rav Nachman and Rava, who rule like R. Shimon regarding after the time, and like R. Yehudah before the time. Even though Rav Nachman cites Rav, and Rav rules like R. Yehudah also after the time, Rav Nachman himself rules like R. Shimon after the time.

Sha'arei Teshuvah (1): A Chacham wanted to say that the Aseh of Tashbisu is only on Erev Pesach, but not during Pesach. Noda bi'Yehudah (2 OC 60,61) rejected this, for one who checks during Pesach blesses. The Berachah was enacted for doing the Mitzvah, and not on avoiding the Isur.

Mishnah Berurah (1): Several Poskim agree that even though Chametz is Asur b'Hana'ah from midday, Bal Yera'eh and Bal Yimatzei do not apply until night, for it says "Shiv'as Yomim Se'or Lo Yimatzei." However, every moment he transgresses Tashbisu.

Shulchan Aruch (ibid.): One may not benefit even from a Nochri's Chametz.

Kaf ha'Chayim (5): Isur v'Heter was unsure whether Chametz of Nochrim is Asur b'Hana'ah. His Safek does not override the Shulchan Aruch's Vadai Isur.

Bi'ur Halachah (DH Afilu): The Acharonim (448) forbid smelling hot bread of Nochrim. Chemed Moshe permits, even regarding a Yisrael's bread, for in YD 108 (Shach 108:27) we permit smelling what is not intended to be smelled. It is not clear to me whether or not this includes bread. The Rema (216:14) brings two opinions about this. See the Gra there. Also, one opinion in YD 108:1 says that since the Isur of Chametz during Pesach is b'Mashehu, even smell is significant. Further, we should forbid lest one come to eat it. Minchas Yakov (36:38) brings like this from the Roke'ach. Above (216:2), in Bi'ur Halachah DH ha'Mosek we wrote like this to forbid, and especially regarding Chametz, which people do not shun the entire year.

Shulchan Aruch (ibid.): Chachamim forbade [eating] it two hours beforehand, i.e. from the start of the fifth hour. The entire fifth hour one may benefit from it. One may sell it to a Nochri. From the beginning of the sixth hour, also Hana'ah from Chametz is forbidden.

Bi'ur Halachah (DH Asruhu): Surely one may not sell it to Nochrim then, for this is Hana'ah. B'Di'eved, if one sold it, the Pri Megadim (MZ 448:5), Binyan Olam and Sho'el u'Meshiv say that it helps at least for after Pesach. I.e. it is not forbidden like Chametz for which one transgressed during Pesach. Some Acharonim say so even if he sold it after six hours, since some Poskim say that there is no Bal Yera'eh until the night. If he cast Chametz into Reshus ha'Rabim after six hours, see 448, Sha'arei Teshuvah, Nishmas Adam and Sedei Chemed regarding whether he may take it after Pesach.

Kaf ha'Chayim (15): Binyan Olam says that selling at a time when it is forbidden mid'Rabanan is like Chametz mid'Rabanan (Nuksheh), which is permitted after Pesach. However, we are stringent for the seller himself.

Kaf ha'Chayim (17): If one is in a different time zone than his Chametz, it depends on the time where the Chametz is (Oneg Yom Tov 36).