Hello (I have not been here for a long time !) .

I suggest you use a structure ===>> example ===>>

Code:
Private Type my_times
  hours As String
  minutes As String
  seconds As String
  milliseconds As String
End Type


Private Sub Commandbutton1_Click()
  Dim time1 As String, time2 As String, milliseconds As Long, my_res As my_times
  time1 = "13:11:25:400"
  time2 = "12:11:26:500"
  milliseconds = to_milliseconds(time1) - to_milliseconds(time2)
  MsgBox from_milliseconds(milliseconds).hours & ":" & from_milliseconds(milliseconds).minutes & ":" & from_milliseconds(milliseconds).seconds & ":" & from_milliseconds(milliseconds).milliseconds
End Sub
Public Function to_milliseconds(what As String) As Long
   toto = Split(what, ":")
   to_milliseconds = (toto(0) * 3600000) + (toto(1) * 60000) + (toto(2) * 1000) + toto(3)
End Function
Private Function from_milliseconds(what As Long) As my_times
  from_milliseconds.hours = Format(what \ 3600000, "00")
  what = what Mod 360000
  from_milliseconds.minutes = Format(what \ 60000, "00")
  what = what Mod 6000
  from_milliseconds.seconds = Format(what \ 1000, "00")
  from_milliseconds.milliseconds = Format(what Mod 1000, "000")
End Function
Should always be right.