Drupal 7.54 DATE_RFC7231 already defined
Are you certain you're running PHP 5 and not a version of PHP 7? I've only ever seen that error when running PHP 7, and it was due to DATE_RFC7231 already defined in PHP 7.0.19 and 7.1.5.

Either way, the patches in that issue were committed and released with 7.55, so it might be worth trying a core update before anything else.