1 private DateTime? _registrationtime;
2 public DateTime? RegistrationTime
3 {
4     set{ _registrationtime=value;}
5     get{return _registrationtime;}
6 }

 1         public static string FormatDateTimeYMD(string date)
 2         {
 3             if (date.Trim() == string.Empty)
 4                 return "";
 5             else
 6                 return DateTime.Parse(date).ToString("yyyy-MM-dd");
 7         }
 8         public static string FormatDateTimeYM(string date)
 9         {
10             if (date.Trim() == string.Empty)
11                 return "";
12             else
13                 return DateTime.Parse(date).ToString("yyyy-MM");
14         }
15         public static DateTime? FormatDateTime(string date)
16         {
17             if (date.Trim() == string.Empty)
18                 return null;
19             else
20                 return DateTime.Parse(date);
21         }

1 model.RegistrationTime=Global.FormatDateTimeYMD(tb_XX.Text.ToString().Trim());