Das lässt sich übrigens vereinfachen zu:

Public Function fcm_xirr(ByVal ZeroValues As Range, ByVal ZeroDates As Range) As Double
     
    Dim i As Long
    Do While ZeroValues.Cells(i + 1).Value = 0
      i = i + 1
    Loop
    
    Set ZeroValues = ZeroValues.Offset(i).Resize(ZeroValues.Cells.Count - i)
    Set ZeroDates = ZeroDates.Offset(i).Resize(ZeroDates.Cells.Count - i)
    
    fcm_xirr = WorksheetFunction.Xirr(ZeroValues, ZeroDates)
    
End Function