How to subtract time in C# - c#-4.0

Question

I am taking two values from database as a string. Now I want to subtract these two values. Please help me and thanks in advance.
private void button3_Click_1(object sender, EventArgs e)
{
label4.Visible = true;
textBox3.Visible = true;
string condur = Properties.Settings.Default.DBConnectionString;
SqlConnection connection = new SqlConnection(condur);
string q1 = "select in_time from in_time where car_reg='" + comboBox1.Text + "' ";
string q2 = "select out_time from out_time where car_reg='" + comboBox1.Text + "' ";
SqlCommand command1 = new SqlCommand(q1, connection);
SqlCommand command2 = new SqlCommand(q2, connection);
try
{
connection.Open();
string q3=command1.ExecuteNonQuery().ToString();
string q4=command2.ExecuteNonQuery().ToString();
DateTime dt1 = DateTime.Parse(q3);
DateTime dt2 = DateTime.Parse(q4);
TimeSpan result = dt2 - dt1; ;
string result1 = result.ToString();
textBox3.Text = result1;
//MessageBox.Show("Insertion successful!");
//textBox1.Text = ""; textBox2.Text = ""; textBox3.Text = ""; comboBox1.Text = ""; comboBox3.Text = ""; textBox11.Text = ""; textBox6.Text = ""; textBox8.Text = ""; textBox9.Text = ""; richTextBox1.Text = ""; textBox4.Text="";
}
catch (Exception exp)
{
throw exp;
}
finally
{
connection.Close();
}
}

Answer 1

It looks like you answered your own question:
TimeSpan result = dt2 - dt1;
To subtract two DateTime values in C#, you simply use the subtraction operator. If there is something wrong with the code sample you posted, you should describe the error, and what you expected to happen.
Edit in response to comment
That exception has nothing to do with subtracting the dates, it has something to do with your conversion from the type string to the type DateTime. You are trying to parse from string to DateTime:
DateTime dt1 = DateTime.Parse(q3);
This fails because q3 does not represent a valid string. The reason why, is the way you (don't) query:
string q3=command1.ExecuteNonQuery().ToString();
ExecuteNonQuery does not return the results of the query, rather it returns the number of changed rows. In other words, ExecuteNonQuery is not intended for querying, but for update scenarios, etc.
What you probably want is to use ExecuteScalar, which returns a single value from the query. If the types in the DB are correct, it will come back as a DateTime, so you don't need to do the DateTime.Parse part. The relevant parts of your code would become:
DateTime q3 = (DateTime)command1.ExecuteScalar();
DateTime q4 = (DateTime)command2.ExecuteScalar();
TimeSpan result = q4-q3;
Oh, and BTW, please look up the terms "SQL Injection", and "parameterized queries" on Google.