Changing date format

I found in calendar.theme that I can change the way that the dates look in each view (day, week, year, etc), so I edited this line (#232):

    case 'DAY':
      $stamp = date_gmmktime(array('year' => $view->year, 'mon' => $view->month, 'mday' => $value));
      return date_format_date('l, F j Y', $stamp);

and left it like this:

    case 'DAY':
      $stamp = date_gmmktime(array('year' => $view->year, 'mon' => $view->month, 'mday' => $value));
      return date_format_date('l, j - F - Y', $stamp);

But it won't work, my calendar still shows the date in the old format in the daily view... any idea why I can't see the change?