string firstDayOfThisQuarter = DateTime.Now.AddMonths(0-(DateTime.Now.Month - 1)%3).ToString("yyyy-MM-01");
Console.WriteLine("本季度的第一天为：{0}",firstDayOfThisQuarter);

 

string lastDayOfThisQuarter = DateTime.Parse(DateTime.Now.AddMonths(3 - ((DateTime.Now.Month - 1)%3)).ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();
string lastDayOfThisQuarter2 = DateTime.Parse(DateTime.Now.AddMonths(3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01")).AddDays(-1).ToString("yyyy-MM-dd");
Console.WriteLine("本季度的最后一天为：{0}", lastDayOfThisQuarter);
Console.WriteLine("本季度的最后一天为：{0}", lastDayOfThisQuarter2);

string firstDayOfNextQuarter = DateTime.Now.AddMonths(3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");
string lastDayOfNextQuarter = DateTime.Parse(DateTime.Now.AddMonths(6 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();
Console.WriteLine("下季度的第一天为：{0}", firstDayOfNextQuarter);
Console.WriteLine("下季度的最后一天为：{0}", lastDayOfNextQuarter);

string firstDayOfLastQuarter = DateTime.Now.AddMonths(-3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");
string lastDayOfLastQuarter = DateTime.Parse(DateTime.Now.AddMonths(0 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();
Console.WriteLine("上季度的第一天为：{0}", firstDayOfLastQuarter);
Console.WriteLine("上季度的最后一天为：{0}", lastDayOfLastQuarter);