The mitzva of getting rid of the ĥametz must be carried out by midday of the fourteenth of Nisan. Every instant that a Jew keeps his ĥametz after that time he is in violation of the positive commandment to remove the ĥametz. Beginning at midday, the Torah prohibition against eating and gaining benefit from ĥametz begins as well (MT, Laws of Ĥametz and Matza 1:8; see also 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 ĥametz for an additional hour. They also forbade eating ĥametz 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 zmanit”). It is forbidden to eat ĥametz from the beginning of the fifth hour; it is forbidden to gain any benefit from the ĥametz from the beginning of the sixth hour; and the Torah prohibition against eating and benefiting from ĥametz begins from the beginning of the seventh hour.
Thus, in practice, it is permissible to eat ĥametz throughout the first four hours of the fourteenth day of Nisan. During the fifth hour, it is forbidden to eat ĥametz by rabbinic decree, but it is still permissible to benefit from the ĥametz – for example, one may feed it to an animal or sell it to a gentile. From the beginning of the sixth hour of the day, it is forbidden by rabbinic decree to gain benefit from the ĥametz. From the time that it is forbidden for a Jew to gain benefit from the ĥametz it is considered as if it does not belong to him, so that he is no longer able to sell it to a gentile or to nullify it. The only way to get rid of it then is to burn it, crumble it and throw it into the sea, or scatter it to the wind (SA 443:1).
However, the poskim disagree about when the day begins. Magen Avraham maintains that it begins at dawn, that is, from when the first light becomes visible in the east. The Vilna Gaon (Gra) 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 approach of Gra. 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 Shaĥarit prayer, whose set time is until the end of the fourth hour (for more detail, see Peninei Halakha: Prayer 11:10 n. 14).
In practice, since the final times for eating and benefiting from ĥametz are of rabbinic origin, as are the sale and bitul of ĥametz, 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. Hilkhot Ĥag Be-ĥag ch. 8 n. 4 summarizes the topic of the Jewish time calculations. 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 situation of large losses, it is permissible to rely on this opinion. In Hilkhot Ĥag Be-ĥag 8:4 in the notes, the author explains what to do in extenuating circumstances where a person forgot to sell or nullify his ĥametz by the end of the fifth hour.