Fix format-time-string %Z bug with negative tz

* src/editfns.c (tzlookup): Fix sign error in %Z when a purely
numeric zone is negative (Bug#28746).
* test/src/editfns-tests.el (format-time-string-with-zone):
Add test for this bug.

2 files changed, 6 insertions, 1 deletions

diff --git a/src/editfns.c b/src/editfns.c
index b03eb947dec..2f8b075817a 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -187,7 +187,8 @@ tzlookup (Lisp_Object zone, bool settz)
                   if (sec != 0)
                     prec += 2, numzone = 100 * numzone + sec;
                 }
-              sprintf (tzbuf, tzbuf_format, prec, numzone,
+              sprintf (tzbuf, tzbuf_format, prec,
+                       XINT (zone) < 0 ? -numzone : numzone,
                        &"-"[XINT (zone) < 0], hour, min, sec);
               zone_string = tzbuf;
             }
diff --git a/test/src/editfns-tests.el b/test/src/editfns-tests.el
index 1c3fde888f6..f910afaf711 100644
--- a/test/src/editfns-tests.el
+++ b/test/src/editfns-tests.el
@@ -166,6 +166,10 @@
     (should (string-equal
              (format-time-string format look '(-28800 "PST"))
              "1972-06-30 15:59:59.999 -0800 (PST)"))
+    ;; Negative UTC offset, as a Lisp integer.
+    (should (string-equal
+             (format-time-string format look -28800)
+             "1972-06-30 15:59:59.999 -0800 (-08)"))
     ;; Positive UTC offset that is not an hour multiple, as a string.
     (should (string-equal
              (format-time-string format look "IST-5:30")