The Mishna says that the zman of krias shema extends for the first 3 hours of the day. To add to yesterdays post, Rabbeinu Yona explains that there is a lichatchila start and finish time for shema, and a bidieved start and finish time. The lichatchila start time of Shema is misheyakir, and the lichatchila finish time is sunrise. From Rabbeinu Yona was see that the importance of davening k'vasikin is not just an advantage of doing tefilla with sunrise, but also getting in shema prior to sunrise which is the lichatchila end time. On a bidieved level, the start time of shema is alos hashachar, and the end time is 3 hours into the day. As I pointed out yesterday, Tosafos seems to contradict themselves (8b - 9b) whether the start time is misheyakir or alos.
It isn't clear when we are given zemanim for shema and tefillah, whether they must be completed by the end time, or is it sufficient to just begin them by the end time. The Magen Avrohom (beginning of Hilchos Tefillah) cited by the M.B. (89:5) who says that the davening must be completed within the first third of the day (z'man tefillah extends 1 halachic hour past z'man krias shema). The Aruch HaShulchan (110:5) infers this from a Magen Avrohom who says that if one sees that the time of tefillah is passing, they should say havineinu. Why not just start the regular shemoneh esrei before the time passes? The M.A. must hold that one has to be finished with the shemoneh esrei before the end of z'man tefillah, so that if he can't he should daven havineinu. The Aruch HaShulchan disagrees and says that it is sufficient to begin tefillah by z'man tefillah, even if one will not be finished until after the time passes. He derives this from Tosafos Brachos 7a that says that Bilam was able to start his curse of the jewish people at the moment of Hashem's anger. Although he couldn't complete his curse in that moment, it was sufficient to just start it. We see from here that once one begins in the "zman", they can finish afterward. The logic of the Aruch HaShulchan would presumably apply to Shema as well. So long as one starts in the right time, they can finish afterward.
It seems to me that from Rabbeinu Yona on our gemara we have a very strong proof to the Magen Avrohom, against the Aruch HaShulchan. Rabbeinu Yona posits that sunrise is the end time for the lichatchila reading of Shema. He explains that the pasuk of ייראוך עם שמש requires that Tefillah be done sometime as the sun is rising and is not a specific momemnt (unlike Rashi who says that the pasuk is referring to shema). If Krias Shema could be done lichatchila after the sun begins to rise, why were the vasikin so meticulous to finish shema exactly upon the sun rising, they could have been less precise and managed to finish shema sometime within the 10 - 15 minutes that it takes the sun to rise and then davened shemone esrei. From this Rabbeinu Yona concludes that their precision was not due to hilchos tefilla, rather to hilchos krias shema. Had they not been so precise, their krias shema would extend beyond the start of sunrise, and they would not fulfill the lichatchila mitzvah of reading shema before sunrise. Rabbeinu Yona then adds, that the vasikin themselves would agree that bidieved one could be yotzei shema for the first 3 hours of the day, even after sunrise, because if they held that sunrise was the absolute end time, they wouldn't have risked losing the entire mitzvah of shema by finishing so close to sunrise. The language of Rabbeinu Yona is:
אם איתא דאינו יוצא בדיעבד אחר הנץ, היאך היו מצמצמין כל כך שיגמרו אותה עם הנץ החמה ממש, היה להם לחוש שמא יעבור זמן ק"ש ולא היו מכניסין ק"ש בספק כזה. Now, if the Aruch HaShulchan was correct, the logic of Rabbeinu Yona wouldn't have made sense. There would be no risk to saying Shema right before sunrise since all we would require is that it begin before sunrise, even if it isn't completed by sunrise. Clearly we see from Rabbeinu Yona that shema would need to be completed prior to the end of the z'man.
Perhaps one could dispute this proof by distinguishing between the z'man of shema and the z'man of tefillah. It is only for tefillah that starting within the time would be okay as suggested by the Aruch HaShulchan whereas shema would need to be completed within its time. However, if for the d'oraysa of shema it would need to be completed within the time, it is more logical that chazal would have modeled the z'man tefillah after the z'man krias shema and wouldn't have been lenient to allow one to just start before the end of the z'man. Therefore it seems that the Magen Avrohom is correct, and the proof of the Aruch Hashulchan from bilam's curse is not the model for chazal's institution of z'man tefillah.