added validation to prevent user from entering invalid 'Authored on' date for 32-bit int

#1778 
The first param for form_set_error should be the name of the form element.  To target your validate function more narrowly can you take a look at adding an additional validate to element_validate for that individual form element (docs)[https://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7#element_validate]