How to get a DateDiff-Value in milliseconds in VBA (Excel)?

If you just need time elapsed in Centiseconds then you don't need the TickCount API. You can just use the VBA.Timer Method which is present in all Office products.

Public Sub TestHarness()
    Dim fTimeStart As Single
    Dim fTimeEnd As Single
    fTimeStart = Timer
    SomeProcedure
    fTimeEnd = Timer
    Debug.Print Format$((fTimeEnd - fTimeStart) * 100!, "0.00 "" Centiseconds Elapsed""")
End Sub

Public Sub SomeProcedure()
    Dim i As Long, r As Double
    For i = 0& To 10000000
        r = Rnd
    Next
End Sub

You could use the method described here as follows:-

Create a new class module called StopWatch Put the following code in the StopWatch class module:

Private mlngStart As Long
Private Declare Function GetTickCount Lib "kernel32" () As Long

Public Sub StartTimer()
    mlngStart = GetTickCount
End Sub

Public Function EndTimer() As Long
    EndTimer = (GetTickCount - mlngStart)
End Function

You use the code as follows:

Dim sw as StopWatch
Set sw = New StopWatch
sw.StartTimer

' Do whatever you want to time here

Debug.Print "That took: " & sw.EndTimer & "milliseconds"

Other methods describe use of the VBA Timer function but this is only accurate to one hundredth of a second (centisecond).