Niddah, Chapter 1, Mishnah 3. The opening chapter of this maseches deals with a woman who discovers blood, and with the question of whether her tumah reaches backward in time. In certain cases the halachah imposes what is called mei'eis le'eis, retroactive tumah covering the previous twenty four hours, so that whatever taharos she handled during that period are treated as temei'im. In other cases the rule is dayah sha'atah, literally that her moment is enough for her: she becomes temeiah from the moment she saw and no earlier. A woman who has a set vesses, meaning that her period arrives at the same fixed time each month, belongs to this second category, since there was no reason to suspect anything before the expected time arrived.
Our mishnah now paints the picture. It asks: "Keitzad dayah sha'atah?" What does it actually look like when we say that a woman's tumah begins only from the moment she sees?
The picture the mishnah draws is an ordinary one. "Haysah yosheves bamitah": a woman is seated upon her bed. "Va'asuka beteharos": her hands are busy with articles that hold the status of taharah. And then, "peirsha vera'asah": she steps away from that work, examines herself, and finds blood.
The ruling is: "Hee temeiah vechulan tehoros." She herself is temeiah, naturally, from that point onward. But all those pure items she had been touching remain exactly as they were: tahor. Since her tumah does not stretch backward at all, there is nothing to cast suspicion on anything she handled beforehand. Every one of those objects keeps its taharah.
Now the mishnah shifts its attention to the second sort of woman, the one whose bleeding does not arrive on schedule and who consequently does bear a tumah that reaches backward in time. Concerning her the mishnah adds a crucial qualification. "Af al pi she'amru": even though the Chachamim declared such a woman to be "metamei mei'eis le'eis", conveying tumah across the full day that preceded her discovery, that backward reach governs nothing more than the fate of taharos she came into contact with during those hours. It has no bearing whatsoever on when her days begin to be numbered. "Eina moneh ela meisha'as shera'asah": her tally of niddah days is reckoned from the actual sighting itself, and not from a day earlier; the clock begins ticking at that instant and runs forward from there.
So the mishnah has given us two precise measures. For the woman whose time is fixed, the moment of seeing is the whole story, and the taharos in her hands stay tahor. And even where retroactive tumah does apply, it is a rule about what came before, never a rule that lengthens her days of niddah.