diff --git a/functions.php b/functions.php
index a0ba2ba..701111b 100644
--- a/functions.php
+++ b/functions.php
@@ -214,3 +214,16 @@ function cbsonline_add_page_to_tag_archive($obj)
         $obj->query_vars['post_type'] = ['post', 'page'];
     }
 }
+
+// override date_format option
+add_filter('option_date_format', 'cbsonline_date_format');
+function cbsonline_date_format($format) {
+    $lang = get_query_var('lang', '');
+    switch($lang) {
+    case 'ja':
+	   return 'Y年n月j日';
+    case 'en_US':
+	   return 'D, d M Y';
+    }
+    return $format;
+}