time-stamp: update support for time zone numeric offset

* time-stamp.el (time-stamp-string-preprocess): Change new format for
numeric time zone from %:z to %5z to match format-time-string better.
(time-stamp-format): Document support for numeric time zone.
See discussion in bug#32931.

* NEWS: Mention time-stamp-format %5z.

3 files changed, 58 insertions, 23 deletions

diff --git a/etc/NEWS b/etc/NEWS
index ff334493298..485d2b1fdf4 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2248,7 +2248,18 @@ and 'gravatar-force-default'.
 *** The built-in ada-mode is now deleted.  The GNU ELPA package is a
 good replacement, even in very large source files.
 
-** Some conversions recommended for 'time-stamp-format' have changed.
+** time-stamp
+
+*** New '%5z' conversion for 'time-stamp-format' gives time zone offset.
+Specifying '%5z' in 'time-stamp-format' or 'time-stamp-pattern'
+expands to the time zone offset, e.g., '+0100'.  The time zone used is
+specified by 'time-stamp-time-zone'.
+
+Because this feature is new in Emacs 27.1, do not use it in the local
+variables section of any file that might be edited by an older version
+of Emacs.
+
+*** Some conversions recommended for 'time-stamp-format' have changed.
 The new documented/recommended %-conversions are closer to those
 used by 'format-time-string' and are compatible at least as far back
 as Emacs 22.1 (released in 2007).
diff --git a/lisp/time-stamp.el b/lisp/time-stamp.el
index 55892cdb83c..4a6b203ccc5 100644
--- a/lisp/time-stamp.el
+++ b/lisp/time-stamp.el
@@ -62,7 +62,8 @@ on the locale setting recorded in `system-time-locale' and
 %02S seconds
 %w   day number of week, Sunday is 0
 %02y 2-digit year: `03'                 %Y  4-digit year: `2003'
-%#Z  lowercase time zone name: `est'    %Z  gives uppercase: `EST'
+%Z   time zone name: `EST'              %#Z gives lowercase: `est'
+%5z  time zone offset: `-0500' (since Emacs 27; see note below)
 
 Non-date items:
 %%   a literal percent character: `%'
@@ -81,7 +82,10 @@ use \"%3a %3b %2d %02H:%02M:%02S %Z %Y\".
 The default padding of some formats has changed to be more compatible
 with format-time-string.  To be compatible with older versions of Emacs,
 specify a padding width (as shown) or use the : modifier to request the
-transitional behavior (again, as shown)."
+transitional behavior (again, as shown).
+
+The behavior of `%5z' is new in Emacs 27.  If your files might be
+edited by older versions of Emacs also, do not use this format yet."
   :type 'string
   :group 'time-stamp
   :version "27.1")
@@ -469,7 +473,7 @@ and all `time-stamp-format' compatibility."
       (cond
        ((eq cur-char ?%)
         ;; eat any additional args to allow for future expansion
-        (setq alt-form nil change-case nil upcase nil field-width "")
+        (setq alt-form 0 change-case nil upcase nil field-width "")
         (while (progn
                  (setq ind (1+ ind))
                  (setq cur-char (if (< ind fmt-len)
@@ -503,7 +507,7 @@ and all `time-stamp-format' compatibility."
           (setq prev-char cur-char)
           ;; some characters we actually use
           (cond ((eq cur-char ?:)
-                 (setq alt-form t))
+                 (setq alt-form (1+ alt-form)))
                 ((eq cur-char ?#)
                  (setq change-case t))
                 ((eq cur-char ?^)
@@ -517,7 +521,7 @@ and all `time-stamp-format' compatibility."
          ((eq cur-char ?%)
           "%")
          ((eq cur-char ?a)              ;day of week
-          (if alt-form
+          (if (> alt-form 0)
                (if (string-equal field-width "")
                    (time-stamp--format "%A" time)
                  "")                    ;discourage "%:3a"
@@ -529,7 +533,7 @@ and all `time-stamp-format' compatibility."
               (time-stamp--format "%#A" time)
             (time-stamp--format "%A" time)))
          ((eq cur-char ?b)              ;month name
-          (if alt-form
+          (if (> alt-form 0)
                (if (string-equal field-width "")
                    (time-stamp--format "%B" time)
                  "")                    ;discourage "%:3b"
@@ -561,7 +565,7 @@ and all `time-stamp-format' compatibility."
          ((eq cur-char ?w)              ;weekday number, Sunday is 0
           (time-stamp--format "%w" time))
          ((eq cur-char ?y)              ;year
-          (if alt-form
+          (if (> alt-form 0)
               (string-to-number (time-stamp--format "%Y" time))
             (if (or (string-equal field-width "")
                     (<= (string-to-number field-width) 2))
@@ -573,9 +577,24 @@ and all `time-stamp-format' compatibility."
          ((eq cur-char ?z)              ;time zone offset
           (if change-case
               ""                        ;discourage %z variations
-            (if alt-form
-                (time-stamp--format "%z" time)
-              (time-stamp--format "%#Z" time)))) ;backward compat, will change
+            (cond ((= alt-form 0)
+                   (if (string-equal field-width "")
+                       (progn
+                         (time-stamp-conv-warn "%z" "%#Z")
+                         (time-stamp--format "%#Z" time))
+                     (cond ((string-equal field-width "1")
+                            (setq field-width "3")) ;%-z -> "+00"
+                           ((string-equal field-width "2")
+                            (setq field-width "5")) ;%_z -> "+0000"
+                           ((string-equal field-width "4")
+                            (setq field-width "0"))) ;discourage %4z
+                     (time-stamp--format "%z" time)))
+                  ((= alt-form 1)
+                   (time-stamp--format "%:z" time))
+                  ((= alt-form 2)
+                   (time-stamp--format "%::z" time))
+                  ((= alt-form 3)
+                   (time-stamp--format "%:::z" time)))))
          ((eq cur-char ?Z)              ;time zone name
           (if change-case
               (time-stamp--format "%#Z" time)
@@ -608,7 +627,7 @@ and all `time-stamp-format' compatibility."
           (system-name))
          ))
         (and (numberp field-result)
-             (not alt-form)
+             (= alt-form 0)
              (string-equal field-width "")
              ;; no width provided; set width for default
              (setq field-width "02"))
@@ -637,7 +656,7 @@ and all `time-stamp-format' compatibility."
 ALT-FORM is whether `#' specified.  FIELD-WIDTH is the string
 width specification or \"\".  TIME is the time to convert."
   (let ((format-string (concat "%" (char-to-string format-char))))
-    (if (and alt-form (not (string-equal field-width "")))
+    (if (and (> alt-form 0) (not (string-equal field-width "")))
         ""                              ;discourage "%:2d" and the like
       (string-to-number (time-stamp--format format-string time)))))
 
diff --git a/test/lisp/time-stamp-tests.el b/test/lisp/time-stamp-tests.el
index 77cd6c5b945..ae8eaf467d9 100644
--- a/test/lisp/time-stamp-tests.el
+++ b/test/lisp/time-stamp-tests.el
@@ -312,19 +312,24 @@
 (ert-deftest time-stamp-test-format-time-zone-offset ()
   "Test time-stamp format %z."
   (with-time-stamp-test-env
-    ;; documented 1995-2019, will change
-    (should (equal (time-stamp-string "%z" ref-time1)
-                   (format-time-string "%#Z" ref-time1 t)))
-    ;; undocumented, changed in 2019
-    (should (equal (time-stamp-string "%:z" ref-time1) "+0000"))
-    (should (equal (time-stamp-string "%:7z" ref-time1) "  +0000"))
-    (should (equal (time-stamp-string "%:07z" ref-time1) "  +0000"))
+    (let ((utc-abbr (format-time-string "%#Z" ref-time1 t)))
+    ;; documented 1995-2019, warned since 2019, will change
+      (time-stamp-should-warn
+       (equal (time-stamp-string "%z" ref-time1) utc-abbr)))
+    ;; implemented and documented (with compat caveat) since 2019
+    (should (equal (time-stamp-string "%5z" ref-time1) "+0000"))
     (let ((time-stamp-time-zone "PST8"))
-      (should (equal (time-stamp-string "%:z" ref-time1) "-0800")))
+      (should (equal (time-stamp-string "%5z" ref-time1) "-0800")))
     (let ((time-stamp-time-zone "HST10"))
-      (should (equal (time-stamp-string "%:z" ref-time1) "-1000")))
+      (should (equal (time-stamp-string "%5z" ref-time1) "-1000")))
     (let ((time-stamp-time-zone "CET-1"))
-      (should (equal (time-stamp-string "%:z" ref-time1) "+0100")))))
+      (should (equal (time-stamp-string "%5z" ref-time1) "+0100")))
+    ;; implemented since 2019, verify that these don't warn
+    (should (equal (time-stamp-string "%-z" ref-time1) "+00"))
+    (should (equal (time-stamp-string "%_z" ref-time1) "+0000"))
+    (should (equal (time-stamp-string "%:z" ref-time1) "+00:00"))
+    (should (equal (time-stamp-string "%::z" ref-time1) "+00:00:00"))
+    (should (equal (time-stamp-string "%:::z" ref-time1) "+00"))))
 
 (ert-deftest time-stamp-test-format-non-date-conversions ()
   "Test time-stamp formats for non-date items."