I had to add this:

if(!class_exists('views_handler_argument_formula')) require '/var/www/BR/drupal.dev/sites/all/modules/views/handlers/views_handler_argument_formula.inc';

to handlers/views_handler_argument_date.inc because I kept getting this error on any /calendar/month/ page:

Fatal error: Class 'views_handler_argument_formula' not found in views_handler_argument_date.inc on line 26

I would rather not leave this code on the page if you have any suggestions.
- Thanks