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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.