How to update a function when opening worksheet

I made

Public Function GetMonth()
    GetMonth = Sheet1.Range("H2").Text
End Function

and then in a cell I entered =GetMonth()
and it displays what I want it to display but does not update when I reopen the program in the even that the date has changed... How would I go about doing this? I don't mind making a button to refresh the data as I already have one that does that and I could just add to it