I also had the same problem. So I searched the web for the answer, but I haven't found it yet. Maybe it's MS's mistake, I think. Instead, I figured out a solution. Here it is.
ReDim CLngArray(UBound(FinishDateArray))
For i = 0 To UBound(FinishDateArray)
CLngArray(i) = CLng(FinishDateArray(i))
Next i
MaxDate = CDate(WorksheetFunction.Max(CLngArray))
MsgBox "Max Date: " & MaxDate