diff --git a/includes/bootstrap.inc b/includes/bootstrap.inc
index 36bd0fa..755c74f 100644
--- a/includes/bootstrap.inc
+++ b/includes/bootstrap.inc
@@ -254,8 +254,13 @@ define('DRUPAL_PHP_FUNCTION_PATTERN', '[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*'
  * http://tools.ietf.org/html/rfc7231#section-7.1.1.1
  *
  * Example: Sun, 06 Nov 1994 08:49:37 GMT
+ *
+ * This constant was introduced in PHP 7.0.19 and PHP 7.1.5 but needs to be
+ * defined by Drupal for earlier PHP versions.
  */
-define('DATE_RFC7231', 'D, d M Y H:i:s \G\M\T');
+if (!defined('DATE_RFC7231')) {
+  define('DATE_RFC7231', 'D, d M Y H:i:s \G\M\T');
+}
 
 /**
  * Provides a caching wrapper to be used in place of large array structures.