The משנה says that if someone makes a נדר to be a נזיר when they have a child and for 100 days, then if they have the child at day 70 or earlier, its not סותר but if they have it after day 70 it is סותר. There is a מחלוקת ראשונים how to understand our משנה. רש"י understand that after day 70, it’s סותר everything done till then (which is actually the גירסא we have in the משנה) and you start all over. תוספות does not have the גירסא of "סותר שבעים" and he learns that if you have the baby before day 70 then the 30 days of נזירות of your son overlaps with your 100 days and you only need to count 100 days. However, if its after day 70, then you pause for the 30 for your son and then resume the days you had left over for your 100 days, with one תגלחת at the end covering both נזירות. Both of these פּשטים have difficulties in the reading or the understanding of the משנה, and especially for תוספות with the words "סותר שאין תגלחת פּחות משלשים". The רמב"ם has a very simple reading. The רמב"ם in הלכות נזירות פּרק ד׳ הל׳ גsays that before day 70, you only need to so the 130 total you had made a נזירות for originally, so you stop on day 70, do your 30 for your son’s נזירות, then have תגלחת, then do 30 for your remaining 100 and do תגלחת again. However, if the child is born after day 70, like at day 80, you would need to break for 30 days and then do an additional 30 days since the רמב"ם insists you always do a תגלחת at the end of theנזירות . So since you cut your hair on day 110, you need to wait till day 140 so it was סותר ten days. This fits the משנה’s words perfectly and are exactly the words of the ירושלמי that תוספות quoted on דף י"ג ע"ב in ד"ה לאחר שבעים and had so much trouble with. ע"ש.

The גמרא asks that רב says that the 30^th day of נזירות is עולה לכאן ולכאן. The גמרא’s מסקנא is that our משנה disagrees with this. The גמרא suggests this is תלוי in the מחלוקת תנאים between רבי יוסי ורבנן about whether a woman who is a שומרת יום כנגד יום and she sees דם halfway through the second day where רבי יוסי says she is both פּטור from פּסח שני and also טמא and the חכמים hold she is חייב בפּסח שני since she is טמא למפרע. Similarly, אבא שאול says the last day of shiva also counts as the first day of shloshim and the תנא קמא disagrees. The מחלוקת is whether we say מקצת היום ככולו or not. How do we put “מקצת היום ככולו” and “עולה לכאן ולכאן” together? After all, we don’t pasken like רבand we say that the 30^th day is not “עולה לכאן ולכאן” yet we still say מקצת היום ככולו in many cases. רב אלחנן זצ"ל in קובץ הערות in סימן ל"ט אות ג explains the יסוד as follows: does מקצת היום ככולו mean that half a day is like a full day, or does it mean each half day counts as a full day? ר׳ יוסי understood that מקצת היום ככולו means that in every day you can have two days, so if she is טהור in the morning and טמא at night, we can say she was טהור one day and then טמא another day. We can also say that the first half of shiva is a full day and the second half is a second day. The חכמים disagree and say that you cant have more than one day per day. All מקצת היום ככולו tells you is that half a day can be stretched and considered to be that the whole day and the day was already finished, but you cant have two statuses in one day.