Multiply date value by Number value

Question

Guys, I'm trying to calculate my total time textbox by the hourly rate textbox. below is an example of how my total time textbox is being output

totalTime.Text = String.Format("{0:D2}:{1:D2}", diff1.Hours, diff1.Minutes);

I have an hour rate textbox that formats by a number like: 45 I want to multiply the hourly rate by the total time, my code below throws an error, and I'm sure it's to do with the formatting?

 int label10 = 0;
            int label11 = 0;

            label10 = Convert.ToInt32(labTxt.Text);
            label11 = Convert.ToInt32(totalTime.Text);

            int total = label10 * label11;
            netLab.Text = total.ToString("F2");