The Shema is a mitzvah d'oraita. So how is it that we can go by the later time of the Gra for sof zman kriat shema rather than the earlier time of the Magen Avraham?
It seems to me that this is a real Safek on a d'oraita which should be l'chumra, and no one should go by the zman of the Gra.
Can anyone explain why people are lenient?