Here's an example (One Way) on how are we going to calculate the time difference between two give time:

C#

        DateTime dFrom;
        DateTime dTo;
        string sDateFrom = "11:56:00";
        string sDateTo = "12:12:00";
        if (DateTime.TryParse(sDateFrom, out dFrom) && DateTime.TryParse(sDateTo, out dTo))
        {
            TimeSpan TS = dTo - dFrom;
            int hour = TS.Hours;
            int mins = TS.Minutes;
            int secs = TS.Seconds;
            string timeDiff = hour.ToString("00") + ":" + mins.ToString("00") + ":" + secs.ToString("00");
            Response.Write(timeDiff); //output 16 mins in format 00:16:00
        } 

VB.NET

Dim dFrom As DateTime
Dim dTo As DateTime
Dim sDateFrom As String = "11:56:00"
Dim sDateTo As String = "12:12:00"
If DateTime.TryParse(sDateFrom, dFrom) AndAlso DateTime.TryParse(sDateTo, dTo) Then
    Dim TS As TimeSpan = dTo - dFrom
    Dim hour As Integer = TS.Hours
    Dim mins As Integer = TS.Minutes
    Dim secs As Integer = TS.Seconds
    Dim timeDiff As String = ((hour.ToString("00") & ":") + mins.ToString("00") & ":") + secs.ToString("00")
    Response.Write(timeDiff)'output 16 mins in format 00:16:00
End If