I'm confused by the Tosafot on page 99b d.h. "Lo Yochal Adam" and the Rosh who apparently quotes an expanded version. If I understand the Tosafot correctly, they say that the Yerushalmi forbids eating any matzah on Erev Pesach (beginning at the sixth hour according to the Rosh) if the matzah would fulfill the mitzvah of matzah at night. Therefore the Mishnah refers to eating "matzah ashira". Such matzah is permitted, says the Tosafot, presumambly meaning it is permitted until the period beginning just before the mincha. (So also "Yachin" on the Mishnah.) "So Rabbenu Tam was accustomed" --suggesting that Rabbenu Tam did not eat "matzah ashira" from just before mincha.
But the Rosh says that Rabbenu Tam was accustomed to eat "matzah ashira" at the se'uda shelishit if Shabbat fell on Erev Pesach. Now that would be after mincha. If so, what was forbidden, according to our Mishnah?
I hope you can clear this up. Thank you.
You are correct that our Mishnah forbids eating from Minchah onwards, but the Gemora on Daf 107b determines that this is Minchah Ketanah which is at 9 1/2 hours of the day. The time for Seudah Shelishis is after Minchah Gedolah which is at 6 1/2 hours. So, presumably Rabeinu Tam ate matzah Ashirah after 6 1/2 hours but before the time forbidden by our Mishnah.
Kol Tuv
Ilan Segal