VBA Function Vs cells operations

Hi,

I`ve got a problem with cells format operations under the Function.
There is the code below and after IF I`d like to clear content of active cell and the cell above or change their color to another but it doesn`t work (but for example merging cells works). The question is why ? It works under Sub but not under Function.
Thanks for help in advance:)

Code:

---

Option Explicit

Function Odliczanie2(data1a As String) As String

Dim data2 As Date
Dim data1 As Date
Dim data2godzina As Integer
Dim data2dzien As Integer
Dim roznicaSekundy As Long

data2 = Now()

If IsDate(data1a) Then
  data1 = CDate(data1a)
  roznicaSekundy = DateDiff("s", data2, data1)
  data2dzien = Int(roznicaSekundy / 86400)
  data2godzina = Round((roznicaSekundy - (data2dzien * 86400)) / 3600)
 
  If roznicaSekundy < 0 Then
    Odliczanie2 = "PO CZASIE !"
  Else
    Odliczanie2 = data2dzien & " d " & data2godzina & " godz "
  End If
Else
  Range(ActiveCell, ActiveCell.Offset(-1, 0)).Clear
                                                            'lub .Interior.ColorIndex = 23
End If


End Function

---

Excell file attached.
For example column "2" cells F12 and F13.

And example attached:)

how are you calling the function?

It`s in the file attached. "=Przykład.xls!Odliczanie2.Odliczanie2(E12)"

changing values to other cells is not possible when called as a worksheet function
as a function it will work if called from someother procedure, but not from a formula

only suggestion i can think of to try is to use application.ontime to call another procedure after a small delay, so that the function can finish first, before clearing the cell