[tz] Re: Use -00 only for invalid time zones

[Paul Eggert via tz]

Thanks for reporting that. I installed the attached; the first is your 
patch with very minor changes; the second documents the change.

I didn't know this this code was used in freebsd-src/contrib/tzcode. I 
now see that there are some minor differences between the FreeBSD copy; 
would it make sense to merge these into tzcode, perhaps with some 
"#ifdef __FreeBSD__"s thrown in? I suspect that would save time overall 
in the long run.From 75e74e90ecdfb463b1a882194716936e38992080 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Dag-Erling=20Sm=C3=B8rgrav?= <d...@freebsd.org>
Date: Mon, 22 Sep 2025 12:10:09 -0700
Subject: [PROPOSED 1/2] Use -00 only for invalid time zones
* localtime.c (tzset_unlocked): If TZ is unset and TZDEFAULT does
not exist, fall back to "UTC" as before, reserving "-00" for when
either TZ or the content of TZDEFAULT is invalid.
---
 localtime.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/localtime.c b/localtime.c
index f534383b..1c6c1a46 100644
--- a/localtime.c
+++ b/localtime.c
@@ -1484,9 +1484,11 @@ tzset_unlocked(void)
     lclptr = sp = malloc(sizeof *lclptr);
 # endif
   if (sp) {
-    if (zoneinit(sp, name, TZLOAD_FROMENV | TZLOAD_TZSTRING) != 0) {
+    int err = zoneinit(sp, name, TZLOAD_FROMENV | TZLOAD_TZSTRING);
+    if (err != 0) {
       zoneinit(sp, "", 0);
-      strcpy(sp->chars, UNSPEC);
+      if (name || err != ENOENT)
+	strcpy(sp->chars, UNSPEC);
     }
     if (0 < lcl)
       strcpy(lcl_TZname, name);
-- 
2.48.1

From a06191238f889c517de6c63b8517f85f29d79f4d Mon Sep 17 00:00:00 2001
From: Paul Eggert <egg...@cs.ucla.edu>
Date: Mon, 22 Sep 2025 12:10:50 -0700
Subject: [PROPOSED 2/2] Add commentary on UTC/-00 patch

---
 NEWS        | 6 ++++++
 localtime.c | 2 ++
 2 files changed, 8 insertions(+)

diff --git a/NEWS b/NEWS
index 4b5fac51..0ad9b891 100644
--- a/NEWS
+++ b/NEWS
@@ -22,6 +22,12 @@ Unreleased, experimental changes
     MANFLAGS can override these defaults.  (Thanks to G. Branden
     Robinson for inspiring these changes.)
 
+  Changes to code
+
+    An unset TZ is no longer invalid when /etc/localtime is missing,
+    and is abbreviated "UTC" not "-00".  This reverts to 2024b behavior.
+    (Problem and patch reported by Dag-Erling Smørgrav.)
+
   Changes to commentary
 
     The leapseconds file contains commentary about the IERS and NIST
diff --git a/localtime.c b/localtime.c
index 1c6c1a46..1226e6e0 100644
--- a/localtime.c
+++ b/localtime.c
@@ -1487,6 +1487,8 @@ tzset_unlocked(void)
     int err = zoneinit(sp, name, TZLOAD_FROMENV | TZLOAD_TZSTRING);
     if (err != 0) {
       zoneinit(sp, "", 0);
+      /* Abbreviate with "-00" if there was an error.
+	 Do not treat a missing TZDEFAULT file as an error.  */
       if (name || err != ENOENT)
 	strcpy(sp->chars, UNSPEC);
     }
-- 
2.48.1