Nazir 6b - Machlokes T"K/ R"E according to R' Masna and Bar Pada

The gemara quotes 2 arguments between the tana kama and r' elazar.

1. stam nazir who becomes tamei on day 30 - t"k: starts nezirus over. r"e: has to wait 7 days to bring korbanos.

2. One who accepts a 100 day nezirus and becomes tamei on day 100 - t"k: starts over. r"e: recounts nezirus for 30 days.

In the opinion of the tana kama, R' Masna understands that we only say miktzah hayom kikulo to enable him to bring korbanos early, but if he becomes tamei before bringing korbanos we don't say miktzas kikulo, therefore he is soser the entire neziurs. Bar pada understands that it is a gezeira d'rabonon.

But in the opinion of R' Elazar, why when he becomes tamei on day 30 (1st case) is he soser only 7 days, but when he becomes tamei on day 100 (2nd case) he is soser 30 days? Tosafos explains that according to Bar pada it works out easily. When he becomes tamei on day 30, the nezirus is over after 29 days so he only has to keep 7 days to become tahor and bring korbanos, but when he becomes tamei on day 100 it is the last day of his nezirus so he should be soser all 100 days if not for the gezeiras hakasuv of reish lakish that teaches he is only soser 30 days.

R' Masna says that R' Elazar holds of miktzas hayom kikulo even for when he becomes tamei before bringing korbanos, therefore when he becomes tamei on day 30 we say miktzah hayom kikulo and his nezirus has already ended, so he only waits out 7 days (by stam nezirus we don't implement the gezeiras hakasuv of reish lakish, only where he accepted a specific number). But when he becomes tamei on day 100, we implement the gezeiras hakasuv of reish lakish that requires a new toras nezirus of 30 days.

The Orach Mishor has an interesting approach to explain R' Masna within R' Eliezer. When one accepts stam nezirus we implement miktzah hayom kikulo, because the number of days he is a nazir is determined by the torah/chazal, therefore we follow there rules which allows for miktzas hayom kikulo. But if one accepts a specific number of days for his nezirus, he means to include all those days fully and we cannot say miktzah hayom kikulo. Therefore, the case of being tamei on day 30 is where he accepted stam neziurs so we say miktzas hayom kikulo and he is now only has to wait 7 days to become clean. But when he accepted 100 day nezirus and becomes tamei on day 100, we don't say miktzah hayom kikulo, rather consider him within the nezirus where he should be soser everything, but we implement the gezeiras hakasuv of reish lakish to reduce his days to be soser only 30 days.