Gemara

Suggestion: Perhaps one may hide [his own] Chametz [so he will not see it]!

Rejection: "Lo Yimatzei."

6a (Rav Yehudah): If one embarks on a ship or with a caravan at least 30 days before Pesach, he need not destroy his Chametz [beforehand]. If he leaves within 30 days, he must destroy it.

(Abaye): Within 30 days he must destroy it only if he intends to return during Pesach. If not, he is exempt.

Objection (Rava): If he intends to return during Pesach, even if he left from Rosh Hashanah he must destroy it!

(Rava): Rather, at least 30 days before he need not destroy it, if he does not intend to return during Pesach. If he intends to return, he must destroy it.

This is like Rava taught elsewhere;

(Rava): If one made his house a storehouse at least 30 days before Pesach, he need not destroy [Chametz underneath the Peros]. If he did so within 30 days, he is obligated;

This is if he does not intend to clear it out during Pesach. If he intends to clear it out during Pesach, he must destroy it.

Question: Why does it depend on 30 days?

Answer (Beraisa): We learn Halachos of Pesach 30 days before Pesach;

R. Shimon ben Gamliel says, we learn them two weeks before Pesach.

Rishonim

Rambam (Hilchos Chametz 2:19): If one embarks on a ship or with a caravan within 30 days, he must check. Before 30 days, he need not check. If he intends to return during Pesach, he must check before leaving, lest he return Erev Pesach Bein ha'Shemashos, and he will not have time to destroy. If he does not intend to return, he need not check.

Rosh (1:10): Really, one should bless Al Bedikas Chametz. It was enacted to bless Al Bi'ur, for immediately after Bedikah he does Bitul, which is Bi'ur, of Chametz he does not know about. He stores known Chametz, and eats it and eradicates it (what remains) from the house in the fifth hour. Bedikah is the beginning of Bi'ur.

Ran (2b DH ha'Mefaresh): Since we learn Halachos of Pesach 30 days before Pesach, the Chiyuv of Bi'ur begins then. Rashi explains that if he intends to return during Pesach, even if he leaves 30 days before he must do Bi'ur, lest he find Chametz in his house when he returns. If so, the Bavli could agree with the Yerushalmi, which says "this is only if he intends to return. If he does not intend to return, even before 30 days he must do Bi'ur." I.e. he is exempt from Bi'ur only if he intends to return before Pesach. If he does not intend to return until Pesach, even 30 days before he must do Bi'ur. Others explain that "he does not intend to return" means that he intends to return only after Pesach. If he intends to return during Pesach or before Pesach, he must do Bi'ur, lest he return Erev Pesach Bein ha'Shemashos, without time to destroy. Also the Rambam says so. The Yerushalmi connotes unlike this.

Ran (DH v'Garsinan): The Yerushalmi says "this is for a Safek. For Vadai, even from Rosh Hashanah." I.e. before 30 days one need not do Bi'ur if it is a Safek, i.e. there is not Vadai Chametz. If there is known Chametz, even from Rosh Hashanah he must do Bi'ur. It seems that the Bavli disagrees. It says "he need not do Bi'ur." It did not say "he need not check." This connotes that before 30 days, even if there is Vadai Chametz he need not eradicate it. It is like Chametz on which a house caved in. It suffices to do Bitul (31b).

Ritva (6a DH Lo): The Ro'oh holds that whenever one must do Bi'ur, he blesses. I say so only within 30 days.

Poskim

Shulchan Aruch (OC 436:1): If one embarks on a ship or with a caravan and does not leave in his house someone who will check it for Chametz, within 30 days, he must check.

Taz (1): This connotes that someone will live there, just no one will check. This is difficult. Surely they will bring Chametz there! How does the Bedikah help? His house acquires for him all Chametz that will be there until Pesach! We must say that he closes the house and does not let anyone bring in Chametz. This is included in his words "anyone to check." Presumably, if someone was living there, he would check!

Magen Avraham (1): He must check the night before he leaves with a Ner. The Taz says that he need not be Mevatel until Erev Pesach, and supports this from Rashi, who says that perhaps he will return too late to be Mevatel. This is no proof. Rashi discusses if he will not do Bi'ur now! Our Gemara discusses Bi'ur of known Chametz. The Shulchan Aruch rules like the Yerushalmi, which requires Bedikah for Safek Chametz. Therefore, one must do also Bitul then. Perhaps he will forget on Erev Pesach!

Mishnah Berurah (3): [Even if he makes a Shali'ach to check] it is proper that also he be Mevatel whenever he is at the time of Bitul.

Kaf ha'Chayim (7): All tasks forbidden [on the night of the 14th] before checking are forbidden [on the night before he leaves, before he checks].

Rema: He does not bless then Al Bi'ur Chametz.

Beis Yosef (DH Kasav, citing Kolbo): Some say that one who embarks blesses when doing Bi'ur [before leaving], even though it is long before Pesach. Others say that he does not bless, since it is not the real time of Bi'ur.

Gra (DH v'Lo): He does not bless, based on how the Rosh explains the enactment of the Berachah. Immediately after Bedikah one does Bitul; he stores known Chametz, and eradicates it from the house in the fifth hour. Here, he does not do Bitul, and he still uses Chametz.

Bi'ur Halachah (DH v'Lo): Also the Levush says not to bless. The Bach says to bless within 30 days. The Pri Chodosh says to bless always, since Chachamim obligated him to check. The Magen Avraham, Taz and other Acharonim did not specify. This connotes that they hold like the Rema. However, the Ro'oh holds like the Pri Chodosh, and the Ritva holds like the Bach. What to do in practice requires investigation.

Shulchan Aruch (ibid.): Before 30 days before Pesach, he need not check. (When Pesach comes, he is Mevatel it.)

Magen Avraham (2): The enactment to check does not take effect on him. However, if he has known Chametz, he must eradicate it before he leaves.

Gra (DH Eino): This is like the Rambam, based on the Yerushalmi. It seems that the Bavli disagrees, and so write the Tur and other Poskim (e.g. Ran). The Rambam generally follows the Yerushalmi. The Kolbo, Semag and Semak rule like this (the Yerushalmi).

Hagahah (in Birkas Eliyahu on Bi'ur ha'Gra, 1): All agree that we follow the Bavli against the Yerushalmi! However, whenever it is possible to explain the Bavli like the Yerushalmi, we do so. The Bach says that it is a flimsy inference to say that since the Bavli discussed Bi'ur, before 30 days one is exempt even for Vadai Chametz.

Mishnah Berurah (5): If there is known Chametz, some obligate Bi'ur, and some are lenient.

Taz (2): It seems that he need not do Bitul [now]. He waits until [Erev] Pesach, and is Mevatel [then]. This is clear from Rashi, who says "he does not bless on the Bedikah, since it is not the real time of Bi'ur."

Gra (DH uchshe'Magia): If he would not do Bitul, he would transgress hiding Chametz. In the Reisha, he was Mevatel at the time of Bedikah. In the Seifa, he was not. The Magen Avraham rules like this, unlike the Taz.

Damesek Eliezer: When he does not intend to return, we rely that he will remember to be Mevatel on Erev Pesach. When he intends to return, he relies on being Mevatel when he returns. We are concerned lest he return when he can no longer be Mevatel! In the Reisha, when he leaves within 30 days, he must check before leaving. He is also Mevatel then; one who checks is Mevatel [right afterwards]. In the Seifa, he does not check when leaving, so he is Mevatel on Erev Pesach. This is like the Magen Avraham, unlike the Taz, who says that also in the Reisha he is Mevatel only on Erev Pesach.

Shulchan Aruch (ibid.): If he intends to return before Pesach, he must check before leaving. Perhaps he will return Erev Pesach Bein ha'Shemashos, and not have time to destroy.

Beis Yosef (DH ha'Mefaresh): Rashi explains that if he intends to return during Pesach, even if he leaves 30 days before he must check, lest when he returns there will be Chametz in his house.

Beis Yosef (DH u'Mah she'Chasav Rabeinu): The Gemara and Tur say that we are concerned "even if he leaves from Rosh Hashanah." same concern applies even if he leaves beforehand! Rather, "Rosh Hashanah" means the beginning of the year, i.e. right after Pesach finishes. If he intends to return, even though there is a year until Pesach, he must check. The Rashba (1:223) connotes like this. The Rif, Rambam and Rosh agree. They wrote Stam "if he intends to return" and did not mention Rosh Hashanah.

Beis Yosef (DH v'Davar): The Tur obligates one who intends to return during Pesach. This is like Rashi. The Rambam obligates even one who intends to return before Pesach.

Taz (3): This refers to one who goes to a far place and intends to return one day before Pesach. Then we are concerned lest he return Bein ha'Shemashos. If he can easily return two days before Pesach, we are not concerned lest he be delayed. If we would, there is no end! (One must always be concerned.) Semak says "we are concerned lest he return Bein ha'Shemashos only if he went to the sea or with a caravan to a far place. This is difficult. The Gemara asks "if he intends to return, he should need to do Bi'ur even from Rosh Hashanah!" If the concern is lest he return Erev Pesach, it is unreasonable to be concerned for this when he leaves from Rosh Hashanah and there is much time to return. This is good according to Rashi." I wrote that we discuss one who plans to return one day, or at most two days, before Pesach.

Magen Avraham (3): If someone merely goes on the road, we are not concerned lest he return Bein ha'Shemashos. We are concerned only for one who goes on the sea or with a caravan (Hagahos Maimoniyos). This connotes that one who goes on the sea or with a caravan must check even if he plans to return long before Pesach. Everything is according to the case. Even if he is nearby, if he intends to return a day or two before Pesach, he must check first, lest he delay returning.

Mishnah Berurah (7): All the more so he must check if he intends to return during Pesach.

Mishnah Berurah (8): Even if he was already Mevatel, he must check, lest he return and find Chametz and eat it.

Kaf ha'Chayim (14): If he is unsure whether or not he will return before Pesach, he need not check.