こんな感じの関数が多分シンプル? (defun jst-to-utc (time-str) (format-time-string "%Y-%m-%dT%H:%M:%S%z" (date-to-time time-str) 0)) (jst-to-utc "2025-02-04T21:57:14+09:00") ;; => "2025-02-04T12:57:14+0000" M-x で呼び出し可能にするにはこう (…