Fix decode-time/encode-time roundtrip on macOS

* src/timefns.c (Fencode_time): Ignore DST flag when the zone is
numeric or is a cons, as the doc string says it’s ignored in that
case, and not ignoring it causes encode-time to not invert
decode-time on some platforms (Bug#35502).
* test/src/timefns-tests.el (encode-time-dst-numeric-zone):
New test.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/timefns.c b/src/timefns.c
index 5005c73b7fc..7b5af6a5d24 100644
--- a/src/timefns.c
+++ b/src/timefns.c
@@ -1488,10 +1488,11 @@ usage: (encode-time &optional TIME FORM &rest OBSOLESCENT-ARGUMENTS)  */)
       tm.tm_mon  = check_tm_member (XCAR (a), 1); a = XCDR (a);
       tm.tm_year = check_tm_member (XCAR (a), TM_YEAR_BASE); a = XCDR (a);
       a = XCDR (a);
-      if (SYMBOLP (XCAR (a)))
-        tm.tm_isdst = !NILP (XCAR (a));
+      Lisp_Object dstflag = XCAR (a);
       a = XCDR (a);
       zone = XCAR (a);
+      if (SYMBOLP (dstflag) && !FIXNUMP (zone) && !CONSP (zone))
+        tm.tm_isdst = !NILP (dstflag);
     }
   else if (nargs < 6)
     xsignal2 (Qwrong_number_of_arguments, Qencode_time, make_fixnum (nargs));