DateTime dt = DateTime.Now;  //当前时间

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

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

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

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

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

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

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

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

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