time-stamp: revert recent change to "%04y"

* time-stamp.el (time-stamp-string-preprocess): Revert change to "%04y"
format made 2 weeks ago by commit 0e56883878 (the previous commit to
this file).  Although undocumented, "%04y" was discovered to be in use
in the wild (2016) and had not issued a warning that it would change.
Add a warning that it will change.

* time-stamp-tests.el (time-stamp-test-year-2digit): add test of "%04y"

1 files changed, 7 insertions, 4 deletions

diff --git a/lisp/time-stamp.el b/lisp/time-stamp.el
index 4fb28b2fd37..284dd48d4fc 100644
--- a/lisp/time-stamp.el
+++ b/lisp/time-stamp.el
@@ -545,7 +545,11 @@ and all `time-stamp-format' compatibility."
          ((eq cur-char ?y)              ;year
           (if alt-form
               (string-to-number (time-stamp--format "%Y" time))
-            (string-to-number (time-stamp--format "%y" time))))
+            (if (or (string-equal field-width "")
+                    (<= (string-to-number field-width) 2))
+                (string-to-number (time-stamp--format "%y" time))
+              (time-stamp-conv-warn (format "%%%sy" field-width) "%Y")
+              (string-to-number (time-stamp--format "%Y" time)))))
          ((eq cur-char ?Y)              ;4-digit year
           (string-to-number (time-stamp--format "%Y" time)))
          ((eq cur-char ?z)              ;time zone lower case
@@ -630,8 +634,7 @@ The new forms being recommended now will continue to work then.")
 
 (defun time-stamp-conv-warn (old-form new-form)
   "Display a warning about a soon-to-be-obsolete format.
-Suggests replacing OLD-FORM with NEW-FORM.
-In use before 2019 changes; will be used again after those changes settle."
+Suggests replacing OLD-FORM with NEW-FORM."
   (cond
    (time-stamp-conversion-warn
     (with-current-buffer (get-buffer-create "*Time-stamp-compatibility*")
@@ -640,7 +643,7 @@ In use before 2019 changes; will be used again after those changes settle."
           (progn
             (insert
              "The formats recognized in time-stamp-format will change in a future release\n"
-             "to be compatible with the new, expanded format-time-string function.\n\n"
+             "to be more compatible with the format-time-string function.\n\n"
              "The following obsolescent time-stamp-format construct(s) were found:\n\n")))
       (insert "\"" old-form "\" -- use " new-form "\n"))
     (display-buffer "*Time-stamp-compatibility*"))))