Dim zeit1 As Date
        Dim zeit2 As Date
        Dim zeitdiff As TimeSpan
        zeit1 = Now
        zeit2 = Now.AddMinutes(300)
        zeitdiff = DateTime.op_Subtraction(zeit1, zeit2)

________
Alle Angaben ohne Gewähr. Keine Haftung für Vorschläge, Tipps oder sonstige Hilfe, falls es schiefgeht, nur Zeit verschwendet oder man sonst nicht zufrieden ist

Dim starttime_milliseconds As String
Dim time_milliseconds As String
Dim time_rest As String
Dim result_milliseconds As String
 
time = Format$(Now, "11:18:17:234")
starttime = Format$(Now, "07:48:22:754")
 
starttime_milliseconds = (CInt(Mid(starttime, 1, 2)) * 3600000) + (CInt(Mid( _
  starttime, 4, 2)) * 60000) + (CInt(Mid(starttime, 7, 2)) * 1000) + CInt(Mid( _
  starttime, 10, 3))
time_milliseconds = (CInt(Mid(time, 1, 2)) * 3600000) + (CInt(Mid(time, 4, 2)) _
* 60000) + (CInt(Mid(time, 7, 2)) * 1000) + CInt(Mid(time, 10, 3))
result_milliseconds = time_milliseconds - starttime_milliseconds
time_hours = CInt(result_milliseconds / 3600000)
time_rest = result_milliseconds Mod 3600000
time_minutes = CInt(time_rest / 60000)
time_rest = time_rest Mod 60000
time_seconds = CInt(time_rest / 1000)
time_rest = time_rest Mod 1000
time_milliseconds = CInt(time_rest)
 
If time_minutes < 10 Then
   time_minutes = "0" & CInt(time_minutes)
End If
 
If time_seconds < 10 Then
   time_seconds = "0" & CInt(time_seconds)
End If
 
If time_milliseconds < 100 And time_milliseconds > 9 Then
   time_milliseconds = "0" & CInt(time_milliseconds)
End If
 
If time_milliseconds < 10 Then
   time_milliseconds = "00" & CInt(time_milliseconds)
End If
 
MsgBox(time_hours & ":" & time_minutes & ":" & time_seconds & "," & _
  time_milliseconds)