In MyZmanim (and other luachs) one can find the latest time for reading Shema according to the calculation of the Magen Avraham and Baal HaTanya / Gra. One can (and should) decide which approach to follow of the two. They arguing about which daytime to divide into 12 parts: from alot to tzeit (the broader one) or neitz to shekiya (the tighter one).
I wonder why their argument has rectifications on saying the morning Shema only? Seemingly this is a big machloket which should have bearings on all other halachic times, like saying Mincha, Maariv, accepting Shabbat, etc.