according to the way r zeira understands the mishna which is that the first part is saying theres a mitzvah to interupt the meal, and the second part is saying that if you didn't yo can shake the lulav all day, there is a problem. rav safra's answer dosen't make sense according to this. rv safra said when there will be no time left after he finishes the meal he must stop immediately, but if there will be time he dosen't have to stop. Thats not what r'zeira said!? R zeira said theres a mitzvah even when there is time? so according to r zeira what question is r safra answering?
yaakov shemesh, ny,us
Rebbi Zeira resolves the discrepancy between the Reisha of our Mishnah ('Yitol al Shulchano') and the Seifa ('Yitol Bein ha'Arbayim') by establishing the former to be Lechatchilah and the latter, Bedi'eved.
However (after proving that the Mishnah is speaking about a Mitzvah mid'Rabanan), he reinstates Rav Safra's initial discrepancy between our Mishnah and the Mishnah in Shabbos. Consequently, when Rav Safra differentiates between when there is time and when there is not, he does so with the full support of Rebbi Zeira.
All Rebbi Zeira has really done is to negate Rava's explanation of the discrepancy, and reinstate the original one, as I explained. He does not disagree with Rav Safra.
Be'Virchas Kol Tuv