//指定时间
DateTime dt = new DateTime(2016, 6, 1);

//当前时间
DateTime dt = DateTime.Now;

//本周周一
DateTime startWeek = dt.AddDays(1-Convert.ToInt32(dt.DayOfWeek.ToString("d")));

//本周周日
 DateTime endWeek = startWeek.AddDays(6);

//本月月初
DateTime startMonth = dt.AddDays(1 - dt.Day);

//本月月末
DateTime endMonth = dt.AddDays(1 - dt.Day).AddMonths(1).AddDays(-1);

//本月月末
DateTime endMonth = startMonth.AddDays((dt.AddMonths(1) - dt).Days - 1);

//本季度初
DateTime startQuarter = dt.AddMonths(0 - (dt.Month - 1) % 3).AddDays(1 - dt.Day);

//本季度末
DateTime endQuarter = startQuarter.AddMonths(3).AddDays(-1);

//本年年初
DateTime startYear = new DateTime(dt.Year, 1, 1);

//本年年末
DateTime endYear = new DateTime(dt.Year, 12, 31);