The Mishnah in Berachothch2? teaches that one may recite Shema' until the end of the third hour of the day, "for it is the way of the sons of kings to rise by three hours." Why is that the deciding ...
It says in the Gemara that we can say Shma until the third hour since that's when kings get up. What kings are we talking about? Jewish kings weren't around until much later, so Sof Zman Krias Shema ...