Thank you for your willingness to help, during the time I was succeed in the macro.
Sub idotartam()
Dim i As Integer
Dim lastRow As Long
With ActiveSheet
lastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
End With
For i = 3 To lastRow
Cells(i, 7).Value = "=IF(ISNUMBER(RC[-2])=FALSE,"""",IF(ISNUMBER(RC[-1])=FALSE,"""",IF(RC[-1]<RC[-2],""HIBÁS INTERVALLUM!"",IF(AND(RC[-2]<>"""",RC[-1]<>""""),DATEDIF(RC[-2],RC[-1],""D""),IF(AND(RC[-2]<>"""",RC[-1]=""""),"""",IF(AND(RC[-1]<>"""",RC[-2]=""""),"""",IF(AND(RC[-2]="""",RC[-1]=""""),"""",FALSE)))))))"
Next i
End Sub