Surprisingly not everything in early christianity was decided at the first counsel of Nicea. I am pretty sure the Dec 25 date was not decided then either, nor was the Christian or Catholic canon.Of course it's not accurate. The date to celebrate his birth was decided at the council of Nicae in 325AD. Not only that but now biblical historians are saying that his birth is off by 4 calendar years, so that would make it really 2016 now.
They did some important shit, but not this stuff.