Q. When making brachos during bein hashemashos, do they count for the previous day or the next day for meah brachos?
A. Mishna Berura (46: 14) quoting Acharonim (see Beis Yosef and others) begins the count of the one hundred brochos from ‘Hamapil,’ indicating that you count from the prior night (See also Betzel Hachochmo 4: 155) This is the opinion of most Poskim. However, Toras Chaim Sofer (10) maintains that the count starts at the morning and the night follows the day. Piskei Teshuva (16) quotes that the Aderes debates the above.
Betzel Hachochmo (ibid) mentions that HR”Z Auerbach zt’l was in doubt about to which day we attribute the brochos recited during bein hashmoshos. (Halichos Shlomo 22: 45).
Horav Shlomo Miller’s Shlit’a opinion is that since it is a Rabbinical mitzva, in doubt it may be applied to the day that most needs it, for fulfilling one hundred brochos.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a