I am always surprised that so much of the frum community follow the more maikel time for latest time for kriyas sh’man in the morning. Even your website shows the gr”a time, rather than the magen avraham’s. Why is this? Surely sh’ma is a mitzvah d’oraisa and so we should go l’chumra in this sofek b’halacha?
The principle dispute between the poskim is whether times are calculated from dawn (Magen Avraham) or from sunrise (Gra and baal ha-Tanya). Although questions of d’oraiso are involved, Aruch Hashulchan (Orach Chaim 58:14) writes that the leaders of generations past have already brought concrete proofs that the time for krias shema (and for other times of the day) are calculated from sunrise, and not from dawn. Elsewhere (443:5), he adds that the opinion of Magen Avraham has been deferred, and that the custom is to follow the opinion of the Gra. Chazon Ish (Orach Chaim 13:4). also writes that the main opinion le’maaseh is that of the Gra; this is also stated by Minchas Yitzchak (3:71).
The question is therefore not a case of safek d’oraiso, because the halachah follows the Gra and Graz, yet it remains a worthy practice to be stringent for the opinion of Magen Avraham. The custom of following the opinions of Gra and Graz is upheld by (Iggros Moshe, Orach Chaim 1:24), and is also followed by Brisk (Birur Halachah (IV, Orach Chaim, 58).