_README.md

I move this file to my emacs.d, you can find it init-calendar.

init-date.el

;;; Code:
;;; Copy from https://gist.github.com/kanru/1a7c4dc58fda760b4bb4

(eval-when-compile (require 'cl))
(require 'calendar)

(defcustom insert-date-format "%Y-%m-%d"
  "This will use for `insert-date'"
  :type 'string
  :group 'calendar)

(defun calendar-date->time (calendar-date)
  "Convert a date as returned from the calendar to a time"
  (encode-time 0 0 0                   ; second, minute, hour
               (nth 1 calendar-date)   ; day
               (nth 0 calendar-date)   ; month
               (nth 2 calendar-date))) ; year

(defun date-from-calendar ()
  "Insert date from `calendar', it will use `insert-date-format' to format date-string."
  (lexical-let ((date-string nil))
    (set-transient-map
     (let ((map (make-sparse-keymap)))
       (set-keymap-parent map calendar-mode-map)
       (define-key map (kbd "RET")
         (lambda ()
           (interactive)
           (switch-to-buffer calendar-buffer)
           (setf date-string (format-time-string insert-date-format
                                                 (calendar-date->time
                                                  (calendar-cursor-to-date t))))
           (calendar-exit)
           (insert date-string)))
       map)
     (lambda () (null date-string)))
    (save-excursion
      (calendar))))

(defun insert-date (prefix)
  "Insert the current date. With prefix-argument, use `format-time-string' by variable
 `insert-date-format'. With two prefix arguments, use `date-from-calendar' to chose date."
  (interactive "P")
  (cond
   ((not prefix)
    (insert (format-time-string insert-date-format)))
   ((equal prefix '(4))
    (date-from-calendar))))

(global-set-key (kbd "C-c c d") 'insert-date)

(provide 'init-calendar)
;;; init-calendar.el ends here