Emacs: Insert Date Time

Here's a command to insert current date/time.

(require 'ido) ; part of emacs

(defun xah-insert-date ()
  "Insert current date time.
Insert date in this format: yyyy-mm-dd.
If `universal-argument' is called first, prompt for a format to use.
If there's text selection, delete it first.

URL `http://xahlee.info/emacs/emacs/elisp_insert-date-time.html'
version 2020-09-07"
  (interactive)
  (let (($style
         (if current-prefix-arg
             (string-to-number
              (substring
               (ido-completing-read
                "Style:"
                '(
                  "1 → 2018-04-12 Thursday"
                  "2 → 20180412224611"
                  "3 → 2018-04-12T22:46:11-07:00"
                  "4 → 2018-04-12 22:46:11-07:00"
                  "5 → Thursday, April 12, 2018"
                  "6 → Thu, Apr 12, 2018"
                  "7 → April 12, 2018"
                  "8 → Apr 12, 2018"
                  )) 0 1))
           0
           )))
    (when (use-region-p) (delete-region (region-beginning) (region-end)))
    (insert
     (cond
      ((= $style 0)
       ;; "2016-10-10"
       (format-time-string "%Y-%m-%d"))
      ((= $style 1)
       ;; "2018-04-12 Thursday"

       (format-time-string "%Y-%m-%d %A"))
      ((= $style 2)
       ;; "20180412224015"
       (replace-regexp-in-string ":" "" (format-time-string "%Y%m%d%T")))
      ((= $style 3)
       (concat
        (format-time-string "%Y-%m-%dT%T")
        (funcall (lambda ($x) (format "%s:%s" (substring $x 0 3) (substring $x 3 5))) (format-time-string "%z")))
       ;; "2018-04-12T22:45:26-07:00"
       )
      ((= $style 4)
       (concat
        (format-time-string "%Y-%m-%d %T")
        (funcall (lambda ($x) (format "%s:%s" (substring $x 0 3) (substring $x 3 5))) (format-time-string "%z")))
       ;; "2018-04-12 22:46:11-07:00"
       )
      ((= $style 5)
       (format-time-string "%A, %B %d, %Y")
       ;; "Thursday, April 12, 2018"
       )
      ((= $style 6)
       (format-time-string "%a, %b %d, %Y")
       ;; "Thu, Apr 12, 2018"
       )
      ((= $style 7)
       (format-time-string "%B %d, %Y")
       ;; "April 12, 2018"
       )
      ((= $style 8)
       (format-time-string "%b %d, %Y")
       ;; "Apr 12, 2018"
       )
      (t
       (format-time-string "%Y-%m-%d"))))))