本季度、本年度第一天

//本季度第一天

DateTime firstDayOfThisQuarter = Convert.ToDateTime(DateTime.Now.AddMonths(0 - (DateTime.Now.Month - 1) % 3).ToString("yyyy-MM-01"));

//本年度第一天

DateTime firstDayOfThisYear = Convert.ToDateTime(DateTime.Now.ToString("yyyy-01-01"));