BREAKING Change date/identifier pattern

Before it was like 20220531_091625, but now it is 20220531T091625.

I also introduced a new face, in case someone wants to style the time
part separately.

README.org

@@ -86,10 +86,10 @@ Every note produced by Denote follows this pattern:
 : DATE--KEYWORDS--TITLE.org
 
 The =DATE= field represents the date in year-month-day format followed
-by an underscore and the current time in hour-minute-second notation.
-The presentation is compact, with an underscore separating the two
-components.  Like this: =20220531_091625=.  The =DATE= serves as the
-unique identifier of each note.
+by the capital letter =T= (for "time") and the current time in
+hour-minute-second notation.  The presentation is compact:
+=20220531T091625=.  The =DATE= serves as the unique identifier of each
+note.
 
 The =KEYWORDS= field consists of one or more entries demarcated by a
 plus sign (the separator is inserted automatically).  Each keyword is a
@@ -106,9 +106,9 @@ string for the =TITLE= of the file name.
 
 Examples:
 
-: 20220107_124941--economics--plentiful-and-predictable-liquidity.org
-: 20220214_104945--emacs+git--git-patch-formatting.org
-: 20220420_113805--three_word_keyword--this-is-a-test.org
+: 20220107T124941--economics--plentiful-and-predictable-liquidity.org
+: 20220214T104945--emacs+git--git-patch-formatting.org
+: 20220420T113805--three_word_keyword--this-is-a-test.org
 
 While files end in the =.org= extension, the Denote code base does not
 actually depend on org.el and/or its accoutrements.

denote-dired.el

@@ -117,6 +117,11 @@ renaming attachments that the user adds to their notes."
   "Face for file name date in `dired-mode' buffers."
   :group 'denote-dired)
 
+(defface denote-dired-dired-field-time
+  '((t :inherit denote-dired-dired-field-date))
+  "Face for file name time in `dired-mode' buffers."
+  :group 'denote-dired)
+
 (defface denote-dired-dired-field-keywords
   '((default :inherit bold)
     (((class color) (min-colors 88) (background light))
@@ -139,9 +144,10 @@ renaming attachments that the user adds to their notes."
 (defconst denote-dired-font-lock-keywords
   `((,denote--file-regexp
      (1 'denote-dired-dired-field-date)
-     (2 'denote-dired-dired-field-delimiter)
-     (3 'denote-dired-dired-field-keywords)
-     (4 'denote-dired-dired-field-delimiter)))
+     (2 'denote-dired-dired-field-time)
+     (3 'denote-dired-dired-field-delimiter)
+     (4 'denote-dired-dired-field-keywords)
+     (5 'denote-dired-dired-field-delimiter)))
   "Keywords for fontification.")
 
 ;;;###autoload

denote.el

@@ -105,10 +105,10 @@ specifiers."
 
 ;;;; Main variables
 
-(defconst denote--id "%Y%m%d_%H%M%S"
+(defconst denote--id "%Y%m%dT%H%M%S"
   "Format of ID prefix of a note's filename.")
 
-(defconst denote--id-regexp "\\([0-9_]+\\{15\\}\\)"
+(defconst denote--id-regexp "\\([0-9]\\{8\\}\\)\\(T[0-9]\\{6\\}\\)"
   "Regular expression to match `denote--id'.")
 
 (defconst denote--keyword-regexp "\\(--\\)\\([0-9A-Za-z_+]*\\)\\(--\\)"