The mitzva of eliminating ḥametz must be carried out by midday of the 14th of Nisan. Every instant that a Jew keeps his ḥametz after that time he is in violation of the positive commandment to remove the ḥametz. Beginning at midday, the Torah prohibition against eating and gaining benefit from ḥametz begins as well (MT, Laws of Ḥametz and Matza 1:8; above, section 1 of the present chapter).
As discussed above (2:2), in order to distance one further from possibly violating commandments, the Sages added to the prohibitions and forbade gaining benefit from ḥametz for an additional hour. They also forbade eating ḥametz for two extra hours, since on a cloudy day people are likely to err by as much as two hours.
These times are calculated by dividing the day into twelve equal parts, each of which is called “a seasonal hour” (“sha’ah zemanit”). It is forbidden to eat ḥametz from the beginning of the fifth hour; it is forbidden to gain any benefit from the ḥametz from the beginning of the sixth hour; and the Torah prohibition against eating and benefiting from ḥametz begins from the beginning of the seventh hour.
Thus, in practice, it is permissible to eat ḥametz throughout the first four hours of the 14th day of Nisan. During the fifth hour, it is forbidden to eat ḥametz by rabbinic decree, but it is still permissible to benefit from the ḥametz – for example, one may feed ḥametz to an animal or sell it to a gentile. With the onset of the sixth hour of the day, it becomes forbidden by rabbinic decree to gain benefit from the ḥametz, and it is consequently no longer considered in his possession, and he can no longer sell it to a gentile or nullify it. The only way to eliminate it is to burn it, crumble it, and throw it into the sea, or scatter it to the wind (SA 443:1).
The poskim disagree about when to calculate the beginning of the day. Magen Avraham maintains that it begins at dawn, that is, from when the first light becomes visible in the east. The Vilna Gaon maintains that it begins at sunrise, that is, from the time when the sun itself becomes visible in the east. The difference between dawn and sunrise is more than an hour; thus, for every halakha contingent on the hours of the day, calendars list two times. The earlier one in the morning is based on the approach of Magen Avraham, and the later one accords with the Vilna Gaon’s approach. This is true concerning the recitation of the morning Shema, which must be done by the end of the first three hours of the day, and it is also true of the Shaḥarit prayer, whose set time is until the end of the fourth hour (Peninei Halakha: Prayer 11:10, n. 14).
In practice, since the final times for eating and benefiting from ḥametz are of rabbinic origin, as are the sale and bitul of ḥametz, the halakha follows the more lenient view, since in cases of doubt on matters of rabbinic origin the halakha follows the lenient view. Nevertheless, it is better to be stringent when possible (MB 443:8).
The first opinion is quoted in the name of MA, even though the author himself was unsure. See also Rema 443:1 and MB ad loc. 9, stating that according to Terumat Ha-deshen this particular issue is assessed using fixed hours, not seasonal hours, and in a case of significant loss, one may rely on this view.