The גמרא says that if a person becomes a נזיר for two rounds (60 days), he can be מגלח on day 30 and it counts as day one of the next cycle. The גמרא asks what the חידוש is since we already know מקצת היום ככולו? The גמרא answers that the חידוש is that one day can be עולה for two days. There is an important קובץ הערותthat asks a very relevant חקירה for our דף. How does מקצת היום ככולו function? Does it mean that you only really need half a day, or does it mean that half a day is as if it’s the full day. He brings as an example to the latter the following: The רשב"א in כתובות דף ל ע"א says that even though העראה is considered a ביאה, we don’t say a גמר ביאה is like two ביאות since העראה isn’t really a ביאה. Rather, its just considered as if a complete ביאה was done. Therefore, if you actually complete the ביאה , its still only considered one ביאה. The other סברא says that מקצת היום ככולו means only half a day is required to start with. There is a big נפקא מינה between these two possibilities. For example (my example): lets say you have to check a certain amount of produce for bugs and if that’s clean you can say it’s a חֽזקה that the rest are clean and not check them. What if you then kept on checking anyway and it turned out the rest wasn’t clean? Well then you just messed yourself up. The reason is its not that only half the produce needs to be clean of bugs. חזקה just says you can check some and its as if all were checked. Similarly, we say by a זבה that she has to have seven clean days but מק͏צת היום ככולו. That means in theory she can go to the מקוה day seven in the morning. Well what if she is clean in the morning but later sees דם? In that case she is סותר her שבעה נקיים and she becomes טמאה retroactively again. The reason is that in that case מקצת היום ככולו doesn’t mean you only need a half day—rather half a day is as if it’s a full day. The קובץ הערות says that the גמרא on דף ט"ו says that whether we can say that one day of נזירות is עולה לכאן ולכאן is a מחלוקת תנאים. He suggests that the explanation of this מחלוקת is exactly this point—if you hold it can be עולה לכאן ולכאן, it must be that מקצת היום ככולו means you only need half a day. If you hold it can’t act as two days then its because you hold מקצת היום ככולו means half a day is as if it’s a full day but one day can’t be two days! That would also be the פּשט in our גמרא as to what the חידוש is that day 30 can be עולה לכאן ולכאן.