Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 23292)
+++ wp-includes/functions.php	(working copy)
@@ -3262,16 +3262,20 @@
  * @return float|bool
  */
 function wp_timezone_override_offset() {
-	if ( !$timezone_string = get_option( 'timezone_string' ) ) {
-		return false;
+	static $gmt_offset;
+
+	if ( ! isset( $gmt_offset ) ) {
+		$gmt_offset = false;
+		$timezone_string = get_option( 'timezone_string' );
+		if ( $timezone_string ) {
+			$timezone_object = timezone_open( $timezone_string );
+			$datetime_object = date_create();
+			if ( false !== $timezone_object && false !== $datetime_object )
+				$gmt_offset = round( timezone_offset_get( $timezone_object, $datetime_object ) / HOUR_IN_SECONDS, 2 );
+		}
 	}
 
-	$timezone_object = timezone_open( $timezone_string );
-	$datetime_object = date_create();
-	if ( false === $timezone_object || false === $datetime_object ) {
-		return false;
-	}
-	return round( timezone_offset_get( $timezone_object, $datetime_object ) / HOUR_IN_SECONDS, 2 );
+	return $gmt_offset;
 }
 
 /**