Finally I found out.
strftime often used to format nicely date in Japanese, but it will easily broken in production.
There is a workaround to solve this:
function ensureUtf8($text) {
return iconv(mb_detect_encoding(
$text, mb_detect_order(), true), "UTF-8", $text
);
}
function _strftime($format, $time = null) {
return ensureUtf8(strftime($format, $time));
}