erlang:now() -> timestamp()
timestamp() = {MegaSecs, Secs, MicroSecs}
MegaSecs = Secs = MicroSecs = integer() >= 0
MegaSecs兆秒，1兆=1024K,1K=1024 兆秒=1024秒x1024/86400(一天的总秒数)=12.13 天
Secs秒
MicroSecs微妙
获取总毫秒数：1000000000 * MegaSecs + Secs * 1000 + MicroSecs div 1000.
