Dim sFile As String = "d:\mai.txt"
Dim sLines() As String = Split(My.Computer.FileSystem.ReadAllText(sFile), _
  vbCrLf)
 
Dim sumHour As Integer = 0
Dim sumMin As Integer = 0
 
For i As Integer = 1 To sLines.Length - 1
  Dim sData() As String = Split(sLines(i), ";")
  If sData.Length >= 3 Then
    Dim sTime() As String = Split(sData(3), ":")
    sumHour += Val(sTime(0))
    sumMin += Val(sTime(1))
  End If
Next
If sumMin >= 60 Then
  sumHour = sumHour + Int(sumMin / 60)
  sumMin = sumMin Mod 60
End If
 
' Jetzt die neue Zeit in die Datei schreiben
Dim sumLine As String = ";;;" & CStr(sumHour) & ":" & CStr(sumMin) & ":00;" & _
  vbCrLf
My.Computer.FileSystem.WriteAllText(sFile, sumLine, True)

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