Posix / Human Timestamp Elisp (quick and dirty)

cc-timestamp.el

(defun cc/posix-timestamp-to-human (start end)
  (interactive "r")
  (if (use-region-p)
      (let ((regionp (buffer-substring start end)))
        (set 'inputTime (time-convert (string-to-number regionp)))
        (set 'inputBuf (number-to-string (string-to-number regionp)))
        (set 'rfcBuf (format-time-string "%a, %e %b %Y %H:%M:%S %z" inputTime))
        (set 'isoBuf (format-time-string "%Y-%m-%dT%H:%M:%S%z" inputTime))
        (with-output-to-temp-buffer "*timestamps*"
          (princ (concat "| POSIX | " inputBuf " |\n"))
          (princ (concat "| RFC 822 | " rfcBuf " |\n"))
          (princ (concat "| ISO 8601 | " isoBuf " |\n"))
          ))))

(defun cc/human-timestamp-to-posix (start end)
  (interactive "r")
  (if (use-region-p)
      (let ((regionp (buffer-substring start end)))
        (set 'result (number-to-string (time-to-seconds (date-to-time regionp))))
        (kill-new result)
        (with-output-to-temp-buffer "*timestamps*"
          (princ result)))))