Add denote-front-matter-date-format user option

Thanks to Kaushal Modi for proposing the use of an Org timestamp.  (It
was done via email and this information is shared with permission.)

README.org

@@ -331,11 +331,13 @@ Everything is in place to set up the package.
 #+begin_src emacs-lisp
 (require 'denote)
 
+;; Remember to check the doc strings of those variables.
 (setq denote-directory (expand-file-name "~/Documents/notes/"))
 (setq denote-known-keywords
       '("emacs" "philosophy" "politics" "economics"))
 (setq denote-infer-keywords t)
 (setq denote-sort-keywords t)
+(setq denote-front-matter-date-format 'org-timestamp)
 
 (require 'denote-link)
 (require 'denote-dired)
@@ -358,6 +360,18 @@ Everything is in place to set up the package.
                  :jump-to-captured t))))
 #+end_src
 
+* Acknowledgements
+:PROPERTIES:
+:CUSTOM_ID: h:f8126820-3b59-49fa-bcc2-73bd60132bb9
+:END:
+#+cindex: Contributors
+
+Denote is meant to be a collective effort.  Every bit of help matters.
+
++ Author/maintainer :: Protesilaos Stavrou.
+
++ Ideas and/or user feedback :: Kaushal Modi.
+
 * GNU Free Documentation License
 :PROPERTIES:
 :APPENDIX: t

denote.el

@@ -82,6 +82,24 @@ If nil, show the keywords in their given order."
   :group 'denote
   :type 'boolean)
 
+(defcustom denote-front-matter-date-format nil
+  "Date format in the front matter (file header) of new notes.
+
+If the value is nil, use a plain date in YEAR-MONTH-DAY notation,
+like 2022-06-08.
+
+If the value is the `org-timestamp' symbol, format the date as an
+inactive Org timestamp such as: [2022-06-08 Wed 06:19].
+
+If a string, use it as the argument of `format-time-string'.
+Read the documentation of that function for valid format
+specifiers."
+  :type '(choice
+          (const :tag "Just the date like 2022-06-08" nil)
+          (const :tag "An inactive Org timestamp like [2022-06-08 Wed 06:19]" org-timestamp)
+          (string :tag "Custom format for `format-time-string'"))
+  :group 'denote)
+
 ;;;; Main variables
 
 (defconst denote--id "%Y%m%d_%H%M%S"
@@ -288,14 +306,24 @@ Format current time, else use optional ID."
          keywords
          (denote--sluggify title))))
 
+(defun denote--date ()
+  "Expand the date for a new note's front matter."
+  (let ((format denote-front-matter-date-format))
+    (cond
+     ((eq format 'org-timestamp)
+      (format-time-string "[%F %a %R]"))
+     ((stringp format)
+      (format-time-string format))
+     (t (format-time-string "%F")))))
+
 (defun denote--prepare-note (title keywords &optional path)
   "Use TITLE and KEYWORDS to prepare new note file.
 Use optional PATH, else create it with `denote--path'."
   (let* ((p (or path (denote--path title keywords)))
          (default-directory denote-directory)
          (buffer (unless path (find-file p)))
          (header (denote--file-meta-header
-                  title (format-time-string "%F") keywords p
+                  title (denote--date) keywords p
                   (format-time-string denote--id))))
     (unless path
       (with-current-buffer buffer (insert header))