Minor fixes/simplifications to time functions

* doc/lispintro/emacs-lisp-intro.texi (Files List): Simplify.
* doc/lispref/os.texi (Time of Day): Mention format-time-string
as an alternative to current-time-string.
* lisp/arc-mode.el (archive-unixdate, archive-unixtime):
Port better to future versions of Emacs where (COUNT . HZ)
will take precedence to (HI . LO).
* lisp/arc-mode.el (archive-unixtime):
* lisp/calendar/todo-mode.el (todo-insert-item--basic)
(todo-item-done, todo-read-time):
Prefer format-time-string to substringing current-time-string.
* lisp/calc/calc-forms.el (calc-time, calcFunc-now):
Prefer decode-time to parsing the output of current-time-string.
* lisp/emacs-lisp/cl-extra.el (cl--random-time):
Prefer encode-time to hashing the output of current-time-string.
* lisp/gnus/gnus-score.el (gnus-score-headers)
(gnus-score-adaptive):
Avoid stringifying and then reparsing timestamp.
* src/timefns.c (Fencode_time): Omit redundant assignment.

doc/lispintro/emacs-lisp-intro.texi

@@ -15599,7 +15599,7 @@ like this:
    (recursive-lengths-list-many-files
     (files-in-below-directory "/usr/local/src/emacs/lisp/"))
    '<)
-  (insert (format "%s" (current-time-string))))
+  (insert (current-time-string)))
 @end ignore
 
 @node Counting function definitions

doc/lispref/os.texi

@@ -1313,9 +1313,10 @@ This function returns the current time and date as a human-readable
 string.  The format does not vary for the initial part of the string,
 which contains the day of week, month, day of month, and time of day
 in that order: the number of characters used for these fields is
-always the same, so you can reliably
-use @code{substring} to extract them.  You should count
-characters from the beginning of the string rather than from the end,
+always the same, although (unless you require English weekday or
+month abbreviations regardless of locale) it is typically more
+convenient to use @code{format-time-string} than to extract
+fields from the output of @code{current-time-string},
 as the year might not have exactly four digits, and additional
 information may some day be added at the end.
 

lisp/arc-mode.el

@@ -637,7 +637,7 @@ the mode is invalid.  If ERROR is nil then nil will be returned."
 
 (defun archive-unixdate (low high)
   "Stringify Unix (LOW HIGH) date."
-  (let* ((time (cons high low))
+  (let* ((time (list high low))
 	 (str (current-time-string time)))
     (format "%s-%s-%s"
 	    (substring str 8 10)
@@ -646,8 +646,7 @@ the mode is invalid.  If ERROR is nil then nil will be returned."
 
 (defun archive-unixtime (low high)
   "Stringify Unix (LOW HIGH) time."
-  (let ((str (current-time-string (cons high low))))
-    (substring str 11 19)))
+  (format-time-string "%H:%M:%S" (list high low)))
 
 (defun archive-get-lineno ()
   (if (>= (point) archive-file-list-start)

lisp/calc/calc-forms.el

@@ -37,13 +37,11 @@
 (defun calc-time ()
   (interactive)
   (calc-wrapper
-   (let ((time (current-time-string)))
+   (let ((time (decode-time)))
      (calc-enter-result 0 "time"
 			(list 'mod
 			      (list 'hms
-				    (string-to-number (substring time 11 13))
-				    (string-to-number (substring time 14 16))
-				    (string-to-number (substring time 17 19)))
+				    (nth 2 time) (nth 1 time) (nth 0 time))
 			      (list 'hms 24 0 0))))))
 
 (defun calc-to-hms (arg)
@@ -1341,16 +1339,15 @@ as measured in the integer number of days before December 31, 1 BC (Gregorian)."
           (math-parse-iso-date-validate isoyear isoweek isoweekday hour minute second)))))
 
 (defun calcFunc-now (&optional zone)
-  (let ((date (let ((calc-date-format nil))
-		(math-parse-date (current-time-string)))))
-    (if (consp date)
-	(if zone
-	    (math-add date (math-div (math-sub (calcFunc-tzone nil date)
-					       (calcFunc-tzone zone date))
-				     '(float 864 2)))
-	  date)
-      (calc-record-why "*Unable to interpret current date from system")
-      (append (list 'calcFunc-now) (and zone (list zone))))))
+  (let ((date (let ((now (decode-time)))
+		(list 'date (math-dt-to-date
+			     (list (nth 5 now) (nth 4 now) (nth 3 now)
+				   (nth 2 now) (nth 1 now) (nth 0 now)))))))
+    (if zone
+	(math-add date (math-div (math-sub (calcFunc-tzone nil date)
+					   (calcFunc-tzone zone date))
+				 '(float 864 2)))
+      date)))
 
 (defun calcFunc-year (date)
   (car (math-date-to-dt date)))

lisp/calendar/todo-mode.el

@@ -1931,7 +1931,7 @@ their associated keys and their effects."
 			     (calendar-current-date) t t))))
 	     (time-string (or (and time (todo-read-time))
 			      (and todo-always-add-time-string
-				   (substring (current-time-string) 11 16)))))
+				   (format-time-string "%H:%M")))))
 	(setq todo-date-from-calendar nil)
 	(find-file-noselect file 'nowarn)
 	(set-window-buffer (selected-window)
@@ -2881,8 +2881,7 @@ visible."
              (not marked))
       (let* ((date-string (calendar-date-string (calendar-current-date) t t))
 	     (time-string (if todo-always-add-time-string
-			      (concat " " (substring (current-time-string)
-						     11 16))
+			      (format-time-string " %H:%M")
 			    ""))
 	     (done-prefix (concat "[" todo-done-string date-string time-string
 				  "] "))
@@ -6091,7 +6090,7 @@ the empty string (i.e., no time string)."
     (while (not valid)
       (setq answer (read-string "Enter a clock time: " nil nil
 				(when todo-always-add-time-string
-				  (substring (current-time-string) 11 16))))
+				  (format-time-string "%H:%M"))))
       (when (or (string= "" answer)
 		(string-match diary-time-regexp answer))
 	(setq valid t)))

lisp/emacs-lisp/cl-extra.el

@@ -438,9 +438,7 @@ as an integer unless JUNK-ALLOWED is non-nil."
 ;; Random numbers.
 
 (defun cl--random-time ()
-  (let* ((time (copy-sequence (current-time-string))) (i (length time)) (v 0))
-    (while (>= (cl-decf i) 0) (setq v (+ (* v 3) (aref time i))))
-    v))
+  (car (encode-time nil t)))
 
 ;;;###autoload (autoload 'cl-random-state-p "cl-extra")
 (cl-defstruct (cl--random-state

lisp/gnus/gnus-score.el

@@ -1501,7 +1501,7 @@ If FORMAT, also format the current score file."
       (when (and gnus-summary-default-score
 		 scores)
 	(let* ((entries gnus-header-index)
-	       (now (date-to-day (current-time-string)))
+	       (now (time-to-days (current-time)))
 	       (expire (and gnus-score-expiry-days
 			    (- now gnus-score-expiry-days)))
 	       (headers gnus-newsgroup-headers)
@@ -2380,7 +2380,7 @@ score in `gnus-newsgroup-scored' by SCORE."
 	       (memq 'word gnus-newsgroup-adaptive))
       (with-temp-buffer
 	(let* ((hashtb (gnus-make-hashtable 1000))
-	       (date (date-to-day (current-time-string)))
+	       (date (time-to-days (current-time)))
 	       (data gnus-newsgroup-data)
 	       word d score val)
 	  (with-syntax-table gnus-adaptive-word-syntax-table

src/timefns.c

@@ -1475,7 +1475,6 @@ usage: (encode-time &optional TIME FORM &rest OBSOLESCENT-ARGUMENTS)  */)
     {
       if (6 < nargs)
 	zone = args[nargs - 1];
-      form = Qnil;
       tm.tm_sec  = check_tm_member (a, 0);
       tm.tm_min  = check_tm_member (args[1], 0);
       tm.tm_hour = check_tm_member (args[2], 0);