Q. Can one apply in case of need the principle of Miktzas Hayom Kekulo, or part of the day counts as the whole day, to end shiva to the night of the seventh day, before davening in the morning as it is usually done? It will make a difference to the Avel who has a necessity to return home, in these pandemic days and an early flight is essential, or for medical reasons.
A. Tur, Shulchan Aruch, Remah (395: 1) and other Poskim maintain that the principle of part of the day counts as all of it, pertains only when daytime is part of it.
However, Pischei Teshuva (ibid) quotes Ridbaz, that when a mitzva is involved, such as learning Torah or having marital relations, even a part of the night counts. Rabbenu Tam (ibid) permits even taking a bath in pressing need.
Horav Aharon Miller Shlit’a opinion is that in case of need one can follow the opinion of the lenient Poskim.
Rabbi A. Bartfeld as advised by Horav Aharon Miller Shlit’a