C# 获取指定时间的周起始时间/月起始时间/年起始时间

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}