NOTE: Most of these time calculations assume the day is either the autumnal or vernal equinox, where there are 12 hours of sunlight and therefore Neitz is at 6:00 AM and Gra's shkiah is 6:00 PM
There is an extremely famous argument on when the start of halachik night (tzeis hakochavim) is. There are two main opinions:
- THe opinion of Rabbeinu Tam, who states it is 72 minutes after sunset (what we consider sunset)
- and the opinion of the Gr"a, Rabbi Eliyahu of Vilna, who states it is 13 minutes and 30 seconds after sunset.
Now, my question is like this:
It is obvious from the words of Rabbeinu Tam (Berachos Daf 2a Tosafos D"H Mai'aimasai) that they (i.e. the people during the times of the Rishonim) recited Krias Shema after Plag Hamincha. This means they prayed at some point after 75 minutes before Rabbeinu Tam's tzeis , meaning that the prayed sometime after 5:57 PM.
Now, Rash"i says (Berachos Daf 2a D"H Ad Sof) that the people prayed before tzeis. This fits with both shitos (i.e. Rabbeinu Tam and Gr"a), because if he held like the Gr"a they davened sometime before 6:13 and a half, and if he held like Rabbeinu Tam they davened sometime before 7:12. This fits with what we said earlier that they for sure davened after 5:57.
Now, the R"i (Berachos Daf 2a Tosafos D"H Mai'aimasai) states that they prayed after the start of Bein HaShmashos  and before Tzeis. This cannot fit with both the Gr"a (for whom the time period would be 6:00 - 6:13:30) and Rabbeinu Tam (for whom it would be 6:58:30 - 7:12).
Now, we cannot prove whom Rashi held like, however we can assume that the R"i held like his Rebbi Rabbeinu Tam, and thus thought that Bein HaShmashos started at 6:58:30. Therefore, the people davened after 6:58, and yet Rashi still states that it is before Tzeis Hakokhavim, therefore seeming to be a proof that Rashi holds like Rabbeinu Tam and not like the Gr"a.
However, it is not a complete proof because it could be that R"i disagrees with Rabbeinu Tam.
Thus, my question is as follows:
Do either Rash"i or R"i either hold like Rabbeinu Tam?
Either way, please back up your answer with sources...
1 It could be that they prayed even earlier if we assume that a sha'ah zemania is calculated from alos until tzeis, but for the purposes of simplicity I will assume from neitz till shekia.
2 He says that they said Shema acc to R' Eliezer in the Braisa on Berachos 2b, who says from Bein HaShmashos