Why don't we say Miktzas Yom k'Kulo by Shabbos or a Ta'anis? For example, if a man keeps Shabbos for 5 minutes that should be enough! Or, if he keeps a fast for 5 minutes, that should also be enough.
Levi, Brooklyn NY
Dear Levi,
Miktzas Yom k'Kulo is mentioned in various places in Shas, all involving waiting for a time span to ellapse.
1) By a Nazir, whether his 30 days is finished on the 30th day in the morning.
2) Rav Yosi holds that the seven count of a Zav is over on the morning of the seventh day.
3) In Bechoros 20, when discussing the 5 month pregnancy term of an animal, whether birth can be at the beginning of the last day.
4) Pesachim 55a, can grafting which takes 3 days to root take root after part of the third day.
5) Various Halachos of mourning (not all) are completed after part of the last day.
In your cases, Shabbos and Ta'annis do not terminate after a set period of time passes. It is Shabbos until an event occurs (it becomes dark) and we are commanded to do the Mitzvos of Shabbos as long as it is Shabbos.
All the best,
Reuven Weiner