dates: MySQL 'GMT' includes BST

The PHP 'GMT' timezone is equivalent to the 'UTC' timezone and therefore
does not include summertime offsets. The 'GMT' timezone reported by MySQL,
however, does use the BST summertime offsets. Therefore, the 'GMT' reported
by MySQL should be explicitly translated to the 'UTC' timezone for use in
PHP.

include/class.timezone.php

@@ -166,12 +166,20 @@ class DbTimezone {
         // Attempt to fetch timezone direct from the database
         $TZ = db_timezone();
 
+        // Translate ambiguous 'GMT' timezone
+        if ($TZ === 'GMT') {
+            // PHP assumes GMT == UTC, MySQL assumes GMT == Europe/London.
+            // To shore up the difference, assuming use of MySQL, use the
+            // timezone in PHP which honors BST (British Summer Time)
+            return 'Europe/London';
+        }
         // Forbid timezone abbreviations like 'CDT'
-        if (!in_array($TZ, array('UTC', 'GMT')) && strpos($TZ, '/') === false)
+        elseif ($TZ !== 'UTC' && strpos($TZ, '/') === false) {
             // Attempt to lookup based on the abbreviation
             if (!($TZ = timezone_name_from_abbr($TZ)))
                 // Abbreviation doesn't point to anything valid
                 return false;
+        }
 
         // SYSTEM does not describe a time zone, ensure we have a valid zone
         // by attempting to create an instance of DateTimeZone()