(setq mew-cite-strings-function
(lambda ()
(let* ((date (mew-cite-get-value mew-date:))
(from (mew-cite-get-value mew-from:))
(cite-date (cond
((and date (string-match mew-time-rfc-regex date))
(format "%s %s %s"
(match-string 1 date)
(match-string 2 date)
(match-string 3 date)))
(t "Unknown")))
(cite-from (or from "Unknown")))
(format "On %s, %s wrote:\n"
cite-date cite-from))))

こんな感じ？めんどくさくなったので日付は適当にいじって。