Need Help.. I Really Do Not Know How To Do Conversion From Millisecond To Text In C#

 private void textBox4_TextChanged(object sender, EventArgs e)
        {
            double rank = Convert.ToDouble(textBox4.Text);

            if (rank < 00:00:15:200) { textBox5.Text = "Excellent"; }
            else if (rank >= 00:00:15:200 && rank < 00:00:16:100) { textBox5.Text = "Above Average"; }
            else if (rank >= 00:00:16:200 && rank < 00:00:18:100) { textBox5.Text = "Average"; }
            else if (rank >= 00:00:18:200 && rank < 00:00:19:300) { textBox5.Text = "Below Average"; }
           
            else if (rank >= 00:00:19:300) { textBox5.Text = "Poor"; }
            else { textBox5.Text = "Unknown - Error"; }
        }