Nazir, Chapter 2, Mishnah 10. Continuing the discussion of the previous mishnah, here too the mishnah deals with someone who accepted upon himself two vows of nezirus, and with the question of how they are to be counted one after the other.
The case:
A man accepted upon himself a nezirus that would take effect when he had a son - a nezirus of thirty days - and in addition he accepted upon himself a nezirus of one hundred days. As in the previous mishnah, he begins by counting his own hundred day nezirus, before his wife gave birth, and in the middle of those hundred days she gives birth to a son.
The son is born within seventy days:
If she gave birth to the son before seventy days of his hundred day nezirus had passed - "lo hifsid klum" - he has lost nothing, since at least thirty days still remain to him. What does he do? He interrupts his count, counts thirty days for his son's nezirus, shaves his head and brings his offerings, and afterward completes what remains. For example: the son was born on the sixtieth day of the hundred day nezirus - he counts thirty days for his son, shaves and brings offerings, and forty days remain for him to complete his own nezirus. There is no loss at all here, since in any case he would have had to wait another forty days.
The son is born after seventy days:
But if she gave birth after seventy days of the first nezirus had passed - for example on the eightieth day, so that only twenty days remain to him - he cannot shave, because "she'ein tiglachas pachos mishloshim yom" - there is no shaving with less than thirty days, meaning that thirty days of hair growth are required. Therefore the days he counted after the seventieth do not help him and he cancels them: the ten days he counted after the seventieth he will have to count again, since he needs a full thirty days.
In summary: Someone who accepted upon himself his son's nezirus and a nezirus of one hundred days for himself, and began counting his own nezirus - if the son was born by the seventieth day, he loses nothing, since the thirty days of the son's nezirus are contained within the days that remain to him; and if the son was born after the seventieth day, he cancels the days he counted beyond the seventieth and counts them again, because there is no shaving with less than thirty days.