Calculate Hours Worked (Military Time Used)

So i was given the task of writing a new employee payroll proggy custom to my employers work. One issue was calculating, on the fly, the hours worked.
For example, one employee clocks in then later logs time out, immidiatly on clicking hours box, they should see a sub total of those current hours worked for that shift. I had alot of trouble gathering information for this and finally just wrote my own code. Do to lack of knowledge this may not be the best sub function in the world, but it does EXACTLY what i needed it to do, and since there is so little out there for help, i figured i would provide it here for others.

to use the sub is easy
select a textbox or richtext to have focus then fill in the blanks. i'll put axample at end

the Sub Code:

Public Sub countHours(ByVal timeIn As String, ByVal timeOut As String, ByVal hours As TextBox, ByVal ooPs As Object)
        Try
            Dim firstTime As Integer = Convert.ToInt16(timeIn)
            Dim secondTime As Integer = Convert.ToInt16(timeOut)
            If secondTime > firstTime Then
                Dim hrsTimeIn As DateTime = DateTime.ParseExact(timeIn, "HHmm", CultureInfo.InvariantCulture)
                Dim hrsTimeOut As DateTime = DateTime.ParseExact(timeOut, "HHmm", CultureInfo.InvariantCulture)
                Dim totalTime As TimeSpan = hrsTimeOut.Subtract(hrsTimeIn)
                hours.Text = totalTime.ToString
            ElseIf firstTime > secondTime Then
                Dim zeroHour As String = "0000"
                Dim tfourHour As String = "2359"
                Dim oneMin As TimeSpan
                oneMin = New TimeSpan(0, 1, 0)
                Dim midnight As DateTime = DateTime.ParseExact(zeroHour, "HHmm", CultureInfo.InvariantCulture)
                Dim twentyFour As DateTime = DateTime.ParseExact(tfourHour, "HHmm", CultureInfo.InvariantCulture)
                Dim hrsTimeIn As DateTime = DateTime.ParseExact(timeIn, "HHmm", CultureInfo.InvariantCulture)
                Dim hrsTimeOut As DateTime = DateTime.ParseExact(timeOut, "HHmm", CultureInfo.InvariantCulture)
                Dim firstDay As TimeSpan = twentyFour.Subtract(hrsTimeIn).Add(oneMin)
                Dim secondDay As TimeSpan = hrsTimeOut.Subtract(midnight)
                Dim totalTime As TimeSpan = firstDay.Add(secondDay)
                hours.Text = totalTime.ToString
            End If
        Catch ex As Exception
            ooPs.focus()
            MsgBox("Incorrect Time Value Entered, " & vbCrLf & "Please Use Military Time. " & vbCrLf & "exp. 1:35PM = 1335")
        End Try
    End Sub

use like this

the Use Code:

Private Sub txtTransHours_GotFocus(ByVal sender As Object, ByVal e As System.EventArgs) Handles txtTransHours.GotFocus
        countHours(txtTransTimeIn.Text, txtTransTimeOut.Text, txtTransHours, txtTransTimeIn)
    End Sub

it works! and it's simple! if anyone has better way, please post it!
PS. this code also works for night shifts! like 11:00P to 7:00A and stuff!

Night shift example
http://i669.photobucket.com/albums/v...tempTime01.png

Night shift example 2
http://i669.photobucket.com/albums/v...tempTime02.png