Time Difference in Formview Insert Item Template

 I have three textboxes and a Calculate Button in a Formview Insert Item Template.
1. StartTime
2. FinishTime
3. TotalTime
4. Calculate Button
I would like it so when someone clicks calculate it automatically calculates the difference between startTime and Finishtime and outputs it into the TotalTime Textbox.
I have StartTime and FinishTime set to AutoPostback. I have the following code which works in Internet Explorer 8 and the latest Firefox but it doesnt work in previous versions. ANY IDEAS.
Protected Sub Button4_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim Start_Time, Finish_Time, Total_Hours, Job_Number As TextBox
        Start_Time = FormView1.FindControl("StartTime")
        Finish_Time = FormView1.FindControl("FinishTime")
     ...