Date/Time: Replace all instances of date() with gmdate().

Use of `date()` in core depends on PHP timezone set to UTC and not changed by third party code (which cannot be guaranteed).

`gmdate()` is functionally equivalent, but is not affected by PHP timezone setting: it's always UTC, which is the exact behavior the core needs.

Props nielsdeblaauw, Rarst.
Fixes #46438. See #44491.

git-svn-id: https://develop.svn.wordpress.org/trunk@45424 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/IXR/class-IXR-server.php

@@ -151,7 +151,7 @@ EOD;
             header('Content-Type: text/xml; charset='.$charset);
         else
             header('Content-Type: text/xml');
-        header('Date: '.date('r'));
+        header('Date: '.gmdate('r'));
         echo $xml;
         exit;
     }