.net関連時間処理問題

DateTime dt = DateTime.Now;  //    
DateTime startWeek = dt.AddDays(1 - Convert.ToInt32(dt.DayOfWeek.ToString("d")));  //    
DateTime endWeek = startWeek.AddDays(6);  //    
DateTime startMonth = dt.AddDays(1 - dt.Day);  //    
DateTime endMonth = startMonth.AddMonths(1).AddDays(-1);  //    
//DateTime endMonth = startMonth.AddDays((dt.AddMonths(1) - dt).Days - 1);  //    
DateTime startQuarter = dt.AddMonths(0 - (dt.Month - 1) % 3).AddDays(1 - dt.Day);  //    
DateTime endQuarter = startQuarter.AddMonths(3).AddDays(-1);  //    
DateTime startYear = new DateTime(dt.Year, 1, 1);  //    
DateTime endYear = new DateTime(dt.Year, 12, 31);  //    
    、  、  、  、   、     ，  AddDays()、AddMonths()、AddYears()             。
C# datetime   
//       ，                   
//  DayOfWeek          ，                ，       switch        ，         
string[] Day = new string[] { "   ", "   ", "   ", "   ", "   ", "   ", "   " };
string week = Day[Convert.ToInt32(DateTime.Now.DayOfWeek.ToString("d"))].ToString();
//  ，  ，    7 ，         7 ，      
DateTime.Now.AddDays(Convert.ToInt32 (1 - Convert.ToInt32(DateTime.Now.DayOfWeek)) - 7);        //   
DateTime.Now.AddDays(Convert.ToInt32 (1 - Convert.ToInt32(DateTime.Now.DayOfWeek)) - 7).AddDays(6);     //   （   ）
//  
DateTime.Now.AddDays(Convert.ToInt32 (1 - Convert.ToInt32(DateTime.Now.DayOfWeek)) + 7);        //   
DateTime.Now.AddDays(Convert.ToInt32(1 - Convert.ToInt32(DateTime.Now.DayOfWeek)) + 7).AddDays(6);      //   
//  ,                1 ，               。      
//     
DateTime.Now.Year.ToString() + DateTime.Now.Month.ToString() + "1"; //   
DateTime.Parse(DateTime.Now.Year.ToString() + DateTime.Now.Month.ToString() + "1").AddMonths(1).AddDays(-1).ToShortDateString();//    
//  C# ToString         
DateTime.Now.ToString("yyyy-MM-01");
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(1).AddDays(-1).ToShortDateString();
//   ，      
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(-1).ToShortDateString();
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();
//   ，      
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(1).ToShortDateString();
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(2).AddDays(-1).ToShortDateString();
//7  
DateTime.Now.Date.ToShortDateString();
DateTime.Now.AddDays(7).ToShortDateString();
//7  
DateTime.Now.AddDays(-7).ToShortDateString();
DateTime.Now.Date.ToShortDateString();
//   ， ToString                           
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).ToShortDateString();
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(1).AddDays(-1).ToShortDateString();
//   ，       
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(-1).ToShortDateString();
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddDays(-1).ToShortDateString();
//   
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(1).ToShortDateString();
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(2).AddDays(-1).ToShortDateString();
//   ，           ，            。     ，           ，       
//                 ，                   
DateTime.Now.AddMonths(0 - ((DateTime.Now.Month - 1) % 3)).AddDays(1 - DateTime.Now.Day);
//  ，                   
DateTime.Parse(DateTime.Now.AddMonths(3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();
//   ，        。。。。  
DateTime.Now.AddMonths(3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");
DateTime.Parse(DateTime.Now.AddMonths(6 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();
//   
DateTime.Now.AddMonths(-3 - ((DateTime.Now.Month - 1) % 3)). AddDays(1 - DateTime.Now);
DateTime.Now.AddMonths(0 - ((DateTime.Now.Month - 1) % 3)).AddDays(1 - DateTime.Now.Day).AddDays(-1).ToShortDateString();