Dear Dairy Excerpted and adapted from a shiur by Rav Yosef Jacobovits May 13, 2021…
Laws related to Berachos
When does the day begin for the counting of one hundred brachos? Does one begin counting from sunset, when the halachic day begins, or does one begin counting from the morning?
Poskim, such as the Mishna Berura (46:14), include the bracha of HaMapil (which is recited at night before going to sleep) as one of the beginning brachos of the day, in the count of one hundred brachos. This would indicate that the counting of brachos begins at night, and like most other mitzvos, the time-frame follows the halachic day.
Interestingly, many poskim write that if one davens Maariv on Erev Shabbos before sunset, or bentches at the conclusion of Shalosh Seudos after Tzeis Hakochavim, these brachos are counted toward the hundred brachos of Shabbos. (See Yabia Omer 10:7 and Shevet Halevi 5:23). This is because the halachic day of Shabbos can be extended both before and after Shabbos (tosofos Shabbos), and brachos recited anytime during Shabbos count toward that day.
Sefer Betzeil Hachochma writes that on a regular week night, if one davens Maariv before sunset, these brachos will count toward the earlier day.