Sub Numeriques()
Dim plage As Range
Dim dates As Variant, numériques As Variant
Dim i As Long
Set plage = Sheets("Feuil1").Range("A1").CurrentRegion.Columns(1)
dates = plage.Value
ReDim numériques(1 To UBound(dates))
For i = 1 To UBound(dates)
If IsDate(dates(i, 1)) Then numériques(i) = CLng(Format(dates(i, 1), "yyyymmdd"))
Next
plage.Value = Application.Transpose(numériques)
plage.NumberFormat = "00000000"
End Sub