Dim time1 As String = "12.6.2012 17:50:00"
Dim time2 As String = "12.6.2012 14:25:20"
 
Dim dt1 As Date, dt2 As Date
 
If Date.TryParse(time1, dt1) AndAlso Date.TryParse(time2, dt2) Then
   Dim ts As TimeSpan = dt1 - dt2
   MsgBox(ts.TotalSeconds.ToString & " Sekunden")
End If