Levi asked:
Why don't we say miktzas yom ki'kilo by shabbos or a tannes? For example: If a man keeps shabbos for 5 minutes that should be enough?...or, if he keeps a fast for 5 minutes he should also be enuough..miktzas yom ki'kilo?
Levi, Brooklyn NY
The Kollel replies:
We only apply Miktzas ha'Yom k'Kulo to Halachos which pertain to a day; not to Halachos that apply to a particular day. The logic for this is obvious: after the Miktzas ha'Yom passes, even if it is considered as though you have kept a full day of Shabbos, nevertheless the remaining part of the day is still Shabbos.
D. Zupnik