DDD是该天在一年内的第多少天，d是在一周内第几天，dd是一个月内的
DY ：Day of week abbreviated Mon, Tue, Fri
DAY ：Day of week spelled out Monday, Tuesday, Friday
D ：Day of week (1–7) 1,2,3,4,5,6,7－－注意：每星期的第1天是”星期日“
DD ：Day of month (1–31) 1,2,3,4…31
DDD ：Day of year (1–366) 1,2,3,4…366
摘自：
https://www.cnblogs.com/tider/archive/2009/05/07/1451296.html#undefined