calendar-copy-date-format.el

(defvar iy/calendar-copy-date-format-history '("%Y-%m-%d"))
(defun iy/calendar-copy-date (arg)
  "Copy date under the cursor. Read format from minibuffer if ARG, 
use recently used format if no ARG. See the function `format-time-string' 
for the document of time format string"
  (interactive "P")
  (let ((date (calendar-cursor-to-date t))
        (format (if arg
                    (completing-read
                     "Date Format:"
                     iy/calendar-copy-date-format-history nil nil nil
                     'iy/calendar-copy-date-format-history nil nil)
                  (car iy/calendar-copy-date-format-history)))
        string)
    (setq date (encode-time 0 0 0 (cadr date) (car date) (nth 2 date)))
    (setq string (format-time-string format date))
    (if (eq last-command 'kill-region)
        (kill-append string nil)
      (kill-new string))))

(defun iy/calendar-mode-init ()
  (local-set-key (kbd "w") 'iy/calendar-copy-date))

(add-hook 'calendar-mode-hook 'iy/calendar-mode-init)