Zavim, Chapter 1, Mishnah 6. The first chapter of Zavim revolves around counting re'iyos (flows) and the days on which they occurred, since the tally decides whether a man is simply tamei or is a zav gadol who brings a korban. This final mishnah of the chapter takes that counting into the murkiest hour of the day: bein hashmashos, twilight, when we cannot say with certainty whether we are still in one day or already in the next.
The case and the difficulty
Our case opens with a man who had two flows: one "achas hayom", a plain re'iyah in the course of the day, and then a second one "bein hashmashos", at twilight. Wherein lies the problem? Simply that the timing of the second flow cannot be pinned down. Does twilight belong to the day that is ending or to the night that is beginning? Accordingly, is our count of days two or three? Should the twilight flow have begun before the line and continued past it, it may be assigned to two distinct days, and once we add the earlier daytime flow the total reaches three.
When the straddling is certain
The mishnah rules: "im yadua shemiktzas hare'iyah mehayom u'miktzasah lamachar, vadai lakorban v'latumah" - if it is known that part of the flow belonged to today and part to the following day, and we also have a flow on the adjoining day, then he is definitely obligated in the korban and definitely tamei, because we have three days.
Notice what this teaches. The twilight re'iyah was not a prolonged one at all; it was an ordinary flow, nothing like the drawn out flow that is counted for its length. Yet because it fell across two time periods, it is credited to two days and counts as two.
When the straddling is in doubt
The mishnah continues: "im safeik shemiktzas hare'iyah mehayom u'miktzasah lamachar" - if we are unsure whether the flow really spanned the two days, or whether it all fell within one of them. Here the ruling is split: "vadai latumah, safeik lakorban". He is certainly tamei, since two days are established no matter how we resolve the doubt. But regarding the korban we are in doubt, for we cannot know whether that third day was reached, and three days are needed for the korban.
What does he do in practice? He brings the korban, but the Rambam writes that he does not eat from it, since his obligation itself is only a possibility.
Twilight on two days
The chapter's closing case is a man who had flows on two successive days, "shnei yamim", with each of them occurring at twilight. Here the mishnah declares him "safeik latumah v'lakorban": uncertain in both respects. It all hinges on where those twilight moments are placed. Any bein hashmashos may be the tail end of the departing day or the opening of the incoming one, so that if some portion of each flow lay on either side of the divide, three days emerge and a korban would be owed. Alternatively, they may fall in a way that leaves the days unconnected, and then even the two days required for tumah may be missing. (Three days is the ceiling in this case; there is no way to arrive at four.) Because the question has no resolution, both his tumah and his korban remain undecided.
And the last clause: "achas bein hashmashos, safeik latumah" - if there was only a single flow, at bein hashmashos, the doubt concerns his tumah alone, whether two days were involved. A korban is not even a question here.
The practical upshot of these last cases is that whatever such a man touches, lies upon or sits upon is left with an uncertain status, tamei in doubt, and must be treated accordingly. The chapter that opened with careful counting closes by admitting that twilight will not always let us finish the count.