Insert datetime into databse

Question

string constr = ConfigurationManager.ConnectionStrings["conn"].ConnectionString;

SqlConnection con = new SqlConnection(constr);

con.Open();

string stime = Text_Stime.Text;

string etime = Text_Etime.Text;

DateTime Sdatetime = new DateTime();

Sdatetime = DateTime.ParseExact(Text_Stime.Text, "yyyy-MM-dd hh:mm tt", null);

DateTime Edatetime = new DateTime();

Edatetime = DateTime.ParseExact(Text_Etime.Text, "yyyy-MM-dd hh:mm tt", null);

string insertData = "insert into ExamDetail (ExamName, SubjectName, Duration, StartTime, EndTime, NegativeMark, QusNo, QusMarks) values (@EName, @SubName, @DUR, @ST, @ET, @NM, @QN,@QM)";

SqlCommand com = new SqlCommand(insertData, con);

com.Parameters.AddWithValue("@EName", DropExamName.SelectedItem.Text);

com.Parameters.AddWithValue("@SubName", DropSubName.SelectedItem.Text);

com.Parameters.AddWithValue("@DUR", DropDuration.SelectedItem.Text);

com.Parameters.AddWithValue("@ST", Sdatetime);

com.Parameters.AddWithValue("@ET", Edatetime);

com.Parameters.AddWithValue("@NM", RadioNegativeMarks.SelectedItem.Text);

com.Parameters.AddWithValue("@QN", Text_question_no.Text);

com.Parameters.AddWithValue("@QM", text_question_marks.Text);

com.ExecuteNonQuery();

I am trying to insert datetime into database. But it showing an error "String was not recognized as a valid DateTime." I am using above code. I am using textbox's TextMode="DateTimeLocal".

Please Help me...How can i remove this error?