按月算

DateTime dt1= DateTime.Parse("2011-11-2");
   DateTime dt2= DateTime.Parse("2011-10-1");
    if(dt1.AddMonths(-1)>dt2)
     Console.WriteLine("more than one month");
    else Console.WriteLine("less than one month");

string MonthStart = DateTime.Now.Year.ToString() + "-" + DateTime.Now.Month.ToString() + "-" + "1" + " " + "00:00:00";

string MonthEnd = Convert.ToString(Convert.ToDateTime(MonthStart).AddMonths(1).AddSeconds(-1));