Sof Zman Achilas Chametz-When does it become prohibited to eat Chametz?
The Biblical Sof Zman: It is Biblically forbidden to eat Chametz starting from midday [the start of the 7th hour of the day] of the 14th and onwards, throughout the seven days of Pesach, and one who does so transgresses a negative command.
The Rabbinical Sof Zman: The Sages added two more Zmaniyos hours towards this prohibition, and it is thus Rabbinically forbidden to eat Chametz from the beginning of the 5th hour of the day, which is the end of the 4th hour of the day. [Practically, this is equivalent to the time of Sof Zman Tefilah for Shacharis.]
How to calculate the end time-Zman Baal Hatanya/Gr”a versus Magen Avraham: The morning begins starting from sunrise, and hence at the end of four Zmaniyos hours from sunrise is Sof Zman Achilas Chametz, the end time for eating Chametz. [Practically, this is equivalent to the time of Sof Zman Tefilah for Shacharis, according to the time of the Baal Hatanya and Gr”a. However, some Poskim rule that the hours are calculated starting from Also Hashachar, and hence follow the Sof Zman Achilas Chametz in accordance to the Sof Zman Tefilah for Shacharis, according to the time of the Magen Avraham. Practically, some communities [particularly Sephardim] are initially stringent to stop eating Chametz by the earlier timeframe [i.e. Zman Magen Avraham]. Many communities, however, are accustomed even initially to eat Chametz until the later time frame [i.e. Zman Baal Hatanya/Gr”a]. The Chabad custom follows this opinion, as is the quoted ruling of Admur/Baal Hatanya, as brought above. Nonetheless, the Chabad custom, as was followed by the Chabad Rabbeim, was to stop eating Chametz 45 minutes before the start of the 5th hour. Thus, in conclusion, even according to the Chabad custom to follow the later timeframe [i.e. Zman Baal Hatanya], in actuality one should arrange to finish his breakfast and stop eating Chametz close to, and at times even earlier, than the early time frame [i.e. Zman Magen Avraham] in order to abstain from Chametz 45 minutes before the end time. Nonetheless, if for whatever reason one did not manage to eat, or finish eating, Chametz by the end time of the Baal Hatanya, then one may even initially eat until this point.]
What is it forbidden to eat? Starting from this time, it is forbidden to eat even Rabbinically forbidden forms of Chametz [i.e. Chametz Nuksha; Rabbinical Taaruvos Chametz]. It is forbidden for Ashkenazim to eat even Matzah Ashira.
· It is Biblically forbidden to eat Chametz starting from midday of Erev Pesach. However, it is Rabbinically forbidden to eat Chametz starting from the beginning of the 5th hour from sunrise. Practically, this is equivalent to the time of Sof Zman Tefilah for Shacharis.]
· Some communities [particularly Sephardim] are initially stringent to stop eating Chametz by the Sof Zman Tefila of the Magen Avraham. Many communities, however, are accustomed even initially to eat Chametz until the Sof Zman Tefila of the Baal Hatanya/Gr”a.
· The Chabad custom is to try to arrange to finish breakfast and stop eating Chametz approximately 45 minutes before the Sof Zman Tefila of the Baal Hatanya. Nonetheless, if for whatever reason one did not manage to eat, or finish eating, Chametz by the end time of the Baal Hatanya, then one may even initially eat until this point.
 Admur 431:2 and in Minyan Hamitzvos printed prior to 429; Beis Yosef 443; Rambam Chameizt Umatzah 1:8; Rebbe Yehuda in Pesachim 28a-b; Rosh Pesachim 2:4
Other Talmudic opinions: Some Tananim were of the opinion that the prohibition of eating Chametz begins from the night of the 15th and not on Erev Pesach. [Rebbe Shimon Bar Yochai in Pesachim ibid]
 Rabbinical Chametz: It is prohibited to eat Rabbinical Chametz, beginning from the same time that it is forbidden to eat or benefit from Biblical Chametz. Thus, it is forbidden to eat or benefit from starting from the Biblical time of midday. [Olas Shabbos 443:1; Chok Yosef 443:2; Kaf Hachaim 443:4] Furthermore, due to the Rabbinical decree explained next, it is Rabbinically forbidden to eat even Rabbinical Chametz starting from the 5th hour.
 The source for prohibiting eating from midday: As the verse states “You shall not eat upon it Chametz”, [“it” here] refers to the Pesach sacrifice. Now, the verse is not coming to warn against eating Chametz at the time that one eats the Pesach sacrifice, which is on the night of the 15th, as there is a verse which already states this by saying “do not eat Chametz…”. Rather, the verse is coming to warn against eating Chametz at the time of the sacrifice of the Pesach, which is from midday of the 14th and onwards. [Admur ibid]
 Admur 443:1; Michaber 443:1; Rebbe Yehuda in Mishneh Pesachim 11b
 How to calculate the hours: Shaos Zmaniyios is calculated by dividing the number of minutes of daylight by 12. Thus, the two hours are calculated in accordance to the number of minutes that exist within the 12 hours of daylight within one’s area. Thus, [both in areas where the daylight hours are less than 60 minutes per hour, and] in places where the daylight hours is more than 60 minutes per hour, one follows the Zmaniyos amount of minutes that exist within the hour, and not 60 hour minutes. [Admur 443:4; M”A 443:3; Olas Shabbos 443:1; Chok Yosef 443:7; Chayeh Adam 123:3] For example, if there are 15 hours of daylight in ones area [900 minutes], then each hour contains 75 minutes. The reason behind this calculation is because the daytime which is between sunrise and sunset is always considered to be twelve hours whether the day is long or short. Thus, in the above example one must stop eating Chametz starting from 2 hours and 30 minutes before midday, and must sell the Chametz to the gentile by 1 hour and 15 minutes before midday.
Other opinions in Admur: Some Poskim rule that these hours are [no more than] 60 minutes each, as is the case by average days [as since these two hours are only Rabbinical therefore one may be lenient]. [brought in Admur 443:4]
 The reason: Although Biblically the prohibition to eat and own Chametz only begins from the end of the 6th hour, nevertheless the sages added two more hours towards this prohibition in order to prevent one from transgressing if he gets mixed up on a cloudy day and comes to think that midday is really two hours latter then what it really is. [Admur ibid] The Sages did not prohibit eating Chametz prior to the 5th hour [during the 4th hour] as one will not come to make a mistake in this matter as this is the time of eating for all people [and they thus know the time]. [M”A 443:2; Kaf Hachaim 443:6]
 Admur 443:4; 459:10; Siddur Admur in the section beginning “Zman Kerias Shema” who counts the three hours from sunrise [unlike his ruling in 58:3 and 89:1; See Shiureiy Tziyon p. 75 footnote 16 and Glosses on Siddur of Rav Raskin 30 footnote 123]; Shiltei Giborim on Mordechai Brachos 4:3; Minchas Kohen Mavo Hashemesh 2:6 in name of Rishonim; Levush 233:1 and 266:2; Chok Yaakov; Tosafus Yom Tov Pesachim 3:2; Divrei Chamudos Brachos 4:14; Kneses Hagedola 58:8; Gr”a 459:5; Tzemach Tzedek Chidushim 3 that so is opinion of Admur in Siddur; Aruch Hashulchan 58:14; Igros Kodesh 10:294; Likkutei Sichos 29:378; Igros Moshe 1:24 “The Gr”a and Graz, are the main opinion regarding all matters.”; Chazon Ish 13:4; Orchos Rabbeinu 1:52 in name of Chazon Ish; Teshuvos Vehanhagos 1:56 in name of Brisker Rav; See Pear Hador of Machon Yerushalayim Teshuvah 44 who proves this to be the opinion of the Rambam; M”B 58:4 brings both opinions of M”A and Gr”a without arbitration; See Piskeiy Teshuvos ibid footnote 22; See this article for the full details of this matter:
Other opinions: Some Poskim rule we calculate the hours starting from Alos Hashachar. [M”B 443:8 in name of many Poskim; M”A 233:3; Peri Chadash 89; Levush 233, 267; Admur 58:3 regarding Shema [unlike his ruling in the Siddur and 443:4; 459:10]; 89:1 regarding Davening; Terumas Hadeshen 1; Yad Aaron 58; Elya Raba 58:2; Shalmei Tzibur p. 93; Chaim Sheol 2:38-Ayin; Chesed Lealafim 58:5; Ben Ish Chaiy Vayakhel 4 that so is main opinion; Kitzur SHU”A 17:1; Shesilei Zeisim 58:2; Kaf Hachaim 58:4; Tzemach Tzedek Chidushim 3 brings a proof for this opinion, unlike Admur in Siddur, however, see Igros Kodesh 10:294 that this may not be his actual opinion; M”B 58:4 brings both opinions of M”A and Gr”a without arbitration
 See previous footnote!
 Chida in Chaim Sheol 2:38 that so is the widespread custom; Ben Ish Chaiy Vayakhel 4 that so is custom of Bagdad; Ashel Avraham Butchach 58 that in a time of need one may rely on the later opinion; Maharshag 1:38; Sefer Bein Hashamshos of Rav Y.M. Tukichinsky that so is the custom in all Eretz Yisrael; Yisrael Vihazmanim 1:8 that so is the accepted custom amongst all Israel; Or Letziyon 2:6-1; Halichos Olam Vaeira 1:3 that so is the Sefaradi custom for many generations; Orchos Rabbeinu 1:52 in name of Steipler; Halichos Shlomo 7:12
 Aruch Hashulchan 58:14 “So is the main opinion and so is printed in the Luchos”; Igros Moshe 1:24 “The custom of most communities in Russia and Lita, as well as the custom of the Bnei Hayeshivos, is like the Gr”a and Graz, and so is the main opinion regarding all matters, and only select individuals were stringent in this.”; Chazon Ish 13:4 that many follow the Gr”a even initially; Orchos Rabbeinu 1:52 in name of Chazon Ish that one may follow the Gr”a even initially; Rav Avraham Chaim Naa’h stated [Printed in Yagdil Torah 6:2604] that one may even initially, eat Chametz until the end time of the Baal Hatanya, and one should not put undue pressure on the mothers to feed their children before the earlier end time. He writes that it is a Mitzvah to tell all those who did not manage to finish eating Chametz by the end of the first time, that they can even initially eat until the end of the second time. He concludes that one who desires to be stringent, should do so for himself and not direct anyone else to adapt his stringency.
 Igros Kodesh 10:294; Likkutei Sichos 29:378;
 Admur in Siddur regarding Kerias Shema; Letter of Rav Yaakov Landau ,printed in Kovetz Yagdil Torah N.Y. Vol. 52 p. 150, that so he was instructed by the Rebbe Rashab, in name of his father the Rebbe Maharash, that even by Rabbinical matters one is to precede himself 45 minutes from the end time, and thus he was told by the Rebbe Rashab to stop eating Chametz 3/4 ‘s of an hour before the time; Rav Eli Landa Shlita confirmed with me that this is the practical Chabad custom that is to be followed [although he said 40 minutes before the time, not 45]
 Avraham Chaim Naa’h, printed in Yagdil Torah 6:2604
 Admur 443:1; Chok Yaakov 443:3; Chemed Moshe 443:1; M”B 433:3; Kaf Hachaim 443:7
Other Opinions: Some Poskim rule that one may eat Chametz Nuksha during the 5th hour, until the 6th hour of the day. [Noda Beyehuda 21, brought in M”B and Kaf Hachaim ibid; See Olas Shabbos 443:1; Chok Yosef 443:2; Kaf Hachaim 443:4]
 Admur 444:3; 471:6
Other opinions: Some Poskim rule that even according to Ashkenazim one may eat Matzah Ashira [egg Matzah] until midday. [Noda Beyehuda 21 brought in Kaf Hachaim 443:7] However, practically, the custom is to avoid eating Matzah Ashira beginning from the time that one must stop eating Chametz, which is the beginning of the 5th hour. [Piskeiy Teshuvos 444:3]