TALMUD Shabbat 9:3
[DI "Since it says, 'And it came to pass on the third day when they were sore" (Gen. 34:25).
[E] "How do we know that they tie a red thread on the head of the scapegoat [which is sent forth]?
[F] "Since it says, 'Though your sins be as scarlet, they shall be white as snow" (Is. 1:18)."
[I.A] Said R. Yohanan, "It was from the procedure at Sinai that they derived the rule [at M. 9: 3A; alternatively, the rule in M. Miq. 8: 3]. Moses came down on Tuesday [Lieberman: "on the third (of the month)], saying to them, 'Be ready against the third day, do not come near a woman" (Ex. 19:15). Now whoever ceased to have sexual relations on the third day then may count [L adds: "the third,"] Tuesday night, Wednesday, Wednesday night, Thursday, and as to Thursday night and Friday, if a woman emitted semen on Thursday night [= five spans], she is unclean, but if it was on Friday, she is clean [after five spans]. [In this case there are five spans of cleanness.]
[B] "As to whoever ceased to have sexual relations on Wednesday, we may then count Wednesday, Wednesday night, Thursday, and as to Thursday night and Friday, if a woman emitted semen at night on Thursday night, she would be unclean, while if it was on Friday by day, she is clean. [In this case there are fewer than five spans of cleanness, but it was acceptable.]" [Cf. Lieberman.]
[C] Said R. Yohanan, "This is the arrangement at Sinai. But for future generations, [there will be either] four or six [spans of cleanness]."
[D] R. Aqiba says, "There must always be five such spans, even in the generations to come." [This is now amplified.]
[E] There we have learned: "She who discharges semen on the third day after having intercourse is clean, the words of R. Eleazar b. Azariah. R. Ishmael says, "Sometimes there are four spans [after which the semen loses its efficacy], sometimes five, sometimes six. lshmael wants two spans, a complete night and the day following it, to elapse to render the discharge clean. If intercourse took place at the end of the first day, and the discharge at the beginning of the fourth, two complete days or four spans intervening, it is clean. But if intercourse took place in the Yerusha1mi Shabbat 9:3 morning of the first day, the discharge will not he clean until the beginning of the fourth day, a lapse of five spans. So too if intercourse took place in the evening preceding the first day, the discharge will be clean at the beginning of the fourth day, that is, six such spans.] R. Aqiba says, "They are always five spans [M. Miq. 8:3J-L].
[F] If part of the first span had passed, they reckon that part of the sixth span completes it [T. Miq. 6: 6D].
[G] Lo, R. Ishmael treats a day as a span and a night as a span. R. Aqiba treats a day as a span and a night as a span. Then what is at issue between them?
[H] It is whether spans serve to complete [the necessary period]. R. Ishmael treats part of a span as the whole of it, and R. Aqiba does not treat part of a span as the whole of it.
[I] It has been taught contrary to the view of R. Aqiba: Therefore if part of the first season has passed, they reckon that part of the sixth season completes it [T. Mid. 6:6D].
[J] It has been taught: R. Eleazar b. Azariah says, "A day and a night constitute a span, and part of a span is equivalent to the whole of it."
[K] And thus has it been taught contrary to the view of R. Eleazar b. Azariah, 'There are occasions in which there is a day and any small amount of time in addition, and yet [a woman] is clean. There may be two days lacking a [small] amount of time, and the woman will be unclean." [Hence part of a span is not equivalent to the whole of it.]
[L] "A day and any small amount of time in addition, and she will be clean": What would be a practical illustration of such a case?
[M] If the woman had sexual relations on the eve of the Sabbath prior to sunset, and she emitted semen at the end of the Sabbath after sunset, lo, we have a full day and just a bit more time, and yet she is clean.
[N] "Two days less any amount of time, and she will be unclean": What would be a practical illustration of such a case?
 If a woman had sexual relations on the eve of the Sabbath after sunset, and she emitted semen on Sunday prior to sunset, lo, we have two days lacking only a small amount of time, and yet she is unclean.
Palestinian (Jerusalem) Talmud, Tract Sabbat, vol.11,