Partial Day as a Minimizer

The Gemora (Nazir 5b) stated that Rav Masna maintains that the Tanna of the Mishna would hold that part of a day is like the entire day and therefore, a nazir may shave on the thirtieth day, even though his nezirus is not complete until the conclusion of the day.

Reb Yosef Engel asks that the Rema Mipano asserts that with respect to things involving sanctity, we do not say that part of a day is like the entire day. This is why it is not sufficient to observe Shabbos and Yom Tov for only part of the day, and on the contrary, we add on in the beginning and upon its conclusion. If so, how could our Gemora say that this principle applies by a nazir? Shouldn’t nezirus be included in halachos involving sanctity?

He answers that since a nazir is referred to as a sinner because of his abstaining from permissible things through a vow, we therefore apply the rule that a part of the day is regarded as the entire day, in order to minimize his sin (by decreasing the amount of time that he would have been required to observe for his nezirus).