以下内容转自http://www.cnblogs.com/musickh/archive/2009/07/16/1524912.html，在此多谢博主！
DateTime.Now=2009-7-16 5:20:56
//今天
DateTime.Now.Date.ToString(); "2009-7-16 0:00:00"
//周起始日期
DateTime.Now.Date.AddDays(-(int)(DateTime.Now.DayOfWeek)+1).ToString(); {2009-7-13 0:00:00} DateTime.Now.Date.AddDays(7-(int)(DateTime.Now.DayOfWeek)).ToString(); {2009-7-19 0:00:00}
//月起始日期 new DateTime(DateTime.Now.Year,DateTime.Now.Month,1).ToString(); {2009-7-1 0:00:00}
new DateTime(DateTime.Now.Date.Year,DateTime.Now.Date.Month,DateTime.DaysInMonth(DateTime.Now.Date.Year,DateTime.Now.Date.Month)).ToString(); "2009-7-31 0:00:00"
//年起始日期 new DateTime(DateTime.Now.Date.Year,1,1).ToString(); {2009-1-1 0:00:00} new DateTime(DateTime.Now.Date.Year,12,DateTime.DaysInMonth(DateTime.Now.Date.Year,DateTime.Now.Date.Month)).ToString(); {2009-12-31 0:00:00}