נדה א׳Niddah 1

Mishnah 1א׳

שַׁמַּאי אוֹמֵר, כָּל הַנָּשִׁים דַּיָּן שְׁעָתָן. הִלֵּל אוֹמֵר, מִפְּקִידָה לִפְקִידָה, וַאֲפִלּוּ לְיָמִים הַרְבֵּה. וַחֲכָמִים אוֹמְרִים, לֹא כְדִבְרֵי זֶה וְלֹא כְדִבְרֵי זֶה, אֶלָּא מֵעֵת לְעֵת מְמַעֵט עַל יַד מִפְּקִידָה לִפְקִידָה, וּמִפְּקִידָה לִפְקִידָה מְמַעֶטֶת עַל יַד מֵעֵת לְעֵת. כָּל אִשָּׁה שֶׁיֶּשׁ לָהּ וֶסֶת, דַּיָּהּ שְׁעָתָהּ. וְהַמְשַׁמֶּשֶׁת בְּעִדִּים, הֲרֵי זוֹ כִפְקִידָה, וּמְמַעֶטֶת עַל יַד מֵעֵת לְעֵת וְעַל יַד מִפְּקִידָה לִפְקִידָה:

Shammai says: For all women, their hour [of discovering menstrual blood] suffices [to reckon the period of their impurity from that moment]. Hillel says: [the period of impurity is reckoned retroactively] from [the previous] examination to [the latest] examination, even for many days [i.e. she is assumed to have been impure the entire time]. The Sages say: [the law is] not according to this one nor according to that one, but rather [the period of impurity is reckoned] from that time to that time [on the previous day, i.e. twenty-four hours earlier, when this] lessens [the period] from examination to examination; and [it is reckoned] from [the previous] examination to [the latest] examination [when this] lessens [the period] from that time to that time [on the previous day, twenty-four hours earlier]. For any woman who has a regular cycle, her hour [of discovering menstrual blood] suffices [to reckon the period of her impurity from that moment]. And if a woman uses checking-cloths [literally: witnesses, to clean herself before and after intercourse], this is thereby like an examination, and it lessens [the period of impurity] from [being measured from] that time to that time [i.e. twenty-four hours earlier], or from [the previous] examination to [the latest] examination.

Mishnah 2ב׳

כֵּיצַד דַּיָּהּ שְׁעָתָהּ. הָיְתָה יוֹשֶׁבֶת בַּמִּטָּה וַעֲסוּקָה בְטָהֳרוֹת, וּפֵרְשָׁה וְרָאֲתָה, הִיא טְמֵאָה וְכֻלָּן טְהוֹרוֹת. אַף עַל פִּי שֶׁאָמְרוּ, מְטַמְּאָה מֵעֵת לְעֵת, אֵינָהּ מוֹנָה אֶלָּא מִשָּׁעָה שֶׁרָאָתָה:

How is it that her hour [of discovering menstrual blood] suffices [to reckon her impurity from that moment]? If she was sitting on a bed and was occupied with handling pure things [i.e. foods or vessels], and she left and [then] saw [blood], she is impure, and they are all pure [since her impurity is reckoned only from that moment]. Even though they said [regarding a woman who does not have a regular period]: she renders items impure from that time to that time [i.e. from twenty-fours hours earlier, retroactively], she only counts [her days of menstrual impurity] from the hour she saw [blood].