Let me start with the second point I raised yesterday: does miktzas hayom k’kulo mean that there is a requirement /chiyuv for a full day of observance that can be satisfied by doing only half a day, or does it mean that all that is required to begin with is observance of half a day? Two gemaras suggest two different answers:
1) The gemara (Nazir 5-6) has a debate between Rav Masna and Bar Pada whether stam nezirus is 29 or 30 days long, meaning whether the nazir would shave on day 30 or day 31. However, the gemara concludes that Rav Masna who holds stam nezirus is 30 days (= day 31 is haircut day) also holds that miktzas ha’yom k’kulo; therefore the nazir is yotzei (b’dieved) even if he shaved on day 30. If both opinions hold that a nazir is yotzei if he shaves on day 30, what then is the difference between them? The meforesh (pseudo-Rashi) writes that the difference is in a case where the nazir became tamei on the afternoon of day 30. According to Bar Pada, nezirus ends on day 29; whatever happens on day 30 after the nazir’s shave is irrelevant. According to Rav Masna, day 30 is still part of the cycle of nezirus; if the nazir became tamei that afternoon, even though he already had shaved, he must re-do his nezirus.
If miktzas ha’yom k’kulo means that there is a chiyuv for only part of the day, then why do we care what happens in the afternoon after the nazir’s shave? However, if miktzas hayom k’kulo means that the chiyuv of nezirus encompasses the entire day, just we count observance of part of the day as a kiyum for the entire day, then I think we understand very clearly why becoming tamei in the afternoon undoes that chiyuv and forfeits the nezirus.
2) A woman becomes a zavah obligated to bring korbanos if she sees dam for three straight days. If she sees dam for one day, she must watch herself for one day b’taharah, as a shomeres yom. If she sees dam again on the second day, she is a zavah, but is not yet obligated to bring korbanos; she must watch on the third day, again shomeres yom k’neged yom again. If on the third day she again sees dam, she is a zavah who must bring korbanos. The gemara (Nazir 16a) asks how there could ever be a case of a zavah obligated to bring korbanos according to Rabbi Yosi, who holds of the principle of miktzas hayom k’kulo. Once the second day starts and the woman has not seen dam, miktzas hayom k’kulo – her count of a second day is complete. If she sees dam again, that is a new re’iya and has nothing to do with the past. The gemara answers that the din of zavah according to Rabbi Yosi would occur if she sees dam continuously for three straight days (see the gemara for a second answer as well).
If there is a chiyuv of shemira from dam for the entire day, just we count a shmira of miktzas hayom as sufficient, then I don't understand the conclusion. Just like if a nazir becomes tamei in the afternoon it forfeits the day despite miktzas hayom k'kulo, so too, the shomeres yom should forfeit her shmira of part of the day if the afternoon proves that the day is one of re'iyas dam. It seems from here that miktzas hayom means that all that is required is a shmira of part of the day.
Perhaps I should not expect consistancy. Even though it's the same words "miktzas ha'yom k'kulo", who says that the principle works the same way with respect to nezirus, shomeres yom, and I haven't even thrown in the machlokes Aba Shaul and Chachamim with respect to aveilus? The very fact that we have different Tanaim and Amoraim arguing in these different places about what seems to be the same principle may hint that we are in fact dealing with different animals.
I am not even convinced I have the lomdus here formulated correctly. I have been focussing on whether miktzas hayom means a chovas hagavra of some sort extends for only part of a day or whether the chiyuv extends for the entire day but is fulfilled with a kiyum on only part of the day. But why not look at time instead of the gavra? Perhaps miktzas hayom k'kulo means that we treat the day viz a viz the halacha in question as ending early. In other words, miktzas hayom k'kulo may mean that there is a chiyuv of aveilus (for example) for the entire seventh day of shiva, but the day ends at 9:00 in the morning instead of 7:30 at night.
Sad to disappoint, but I have not yet fully unravelled where we apply miktzas ha'yom. Maybe I can at least flesh out the question and problems next post, bl"n.