Calculating the Days Regarding Greeting a Mourner

How are the three days calculated during which a mourner does not respond to greetings?

The mourning of the first three days is not calculated from the time of death, but by days. Therefore, if the burial took place even shortly before sunset, the day of burial is considered the first day, followed by two full days, after which one can respond to greetings in a soft voice when greeted.

It is written in the Shulchan Aruch, Siman 393, Se'if 1: "A mourner, during the first three days, does not leave the house of mourning nor go to the cemetery;" and the Rema wrote on this: "And part of the third day is considered as the whole (Hagahot Maimoni, Perek 7)." And it is written in the Chatam Sofer, Yoreh De'ah, Siman 190, that in all laws of the three days, we say part of the day is considered as the whole. Therefore, part of the third day is sufficient. However, Rabbi Akiva Eiger, Yoreh De'ah, Siman 380, Se'if 2, wrote regarding the laws of labor for a mourner: "All three first days. That is, the entire third day is included in the prohibition, and here the rule that part of the day is considered as the whole does not apply, as written in Chochmat Adam in Kuntres Matzevet Moshe, Ot 4," and this is also explicitly stated in the Piskei Tosafot (Ot 108). And Chochmat Adam wrote that the words of the Rema, that part of the third day is considered as the whole, apply only to the prohibition of leaving the mourner's house to the cemetery and the house of mourning, meaning only because it is for the sake of a mitzvah, and this is the custom.