The direct effects of Darwin's work are seen much more readily in our modern society than those of Julius Caesar and Saladin. In addition, as a society, we place a lot of value on significant dates, e.g. anniversaries of 10, 100, 1000 years. 2109 and 819 years aren't "significant" in that sense. As for Jesus, many people celebrate his birth every year. And by the way, Jesus was supposedly born between 6 and 4BCE, not 0(B)CE.