Sorting bib entries by year and month

Question

I am trying to change the presort function to sort my bib entries first by year and then by month - but: No success.

I have added a month.sort function like this (similar to the year.sort function that already existed):

FUNCTION {month.sort}
{ month empty$
    { key empty$
        { "to sort, need month or key in " cite$ * warning$
          ""
        }
        { key sortify }
      if$
    }
    { month sort.format.names }
  if$
}

The original/unchanged presort function looks like this:

FUNCTION {presort}
{ type$ "book" = 
  type$ "inbook" = 
  or
    'author.editor.sort
    { type$ "proceedings" =
        'editor.organization.sort
        { type$ "manual" =
            'author.organization.sort
            'year.sort
          if$
        }
      if$
     }
  if$
  "    " 
  *
  author field.or.null
  sort.format.names
  *
  "    "
  *
  title field.or.null
  sort.format.title
  *
  #1 entry.max$ substring$
  'sort.key$ :=
 }

Any idea what I need to change?

Thanks!