A RFC7231 Compliant date.
http://tools.ietf.org/html/rfc7231#section-7.1.1.1
Example: Sun, 06 Nov 1994 08:49:37 GMT
const RFC7231 = 'D, d M Y H:i:s \G\M\T';
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https://api.drupal.org/api/drupal/core!lib!Drupal!Component!Datetime!DateTimePlus.php/constant/DateTimePlus::RFC7231/8.1.x