From 3155f9903ec263d33f375c762a6a346bc8dc92c5 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Sun, 19 Nov 2023 23:30:02 -0800
Subject: time: bug: must subtract gmtoff, not add.

This works fine:

  1> (time-struct-utc 0)
  #S(time year 1970 month 1 day 1 hour 0 min 0 sec 0 wday 4 yday 0
          dst nil gmtoff 0 zone "GMT")
  2> *1.(time-utc)
  0
  3> *1.(time-local)
  28800

But we want the following to return the same results:

  1> (time-struct-local 0)
  #S(time year 1969 month 12 day 31 hour 16 min 0 sec 0 wday 3 yday 364
          dst nil gmtoff -28800 zone "PST")
  2> *1.(time-utc)
  -57600
  3> *1.(time-local)
  -28800

With the patch, we do:

  1> (time-struct-local 0)
  #S(time year 1969 month 12 day 31 hour 16 min 0 sec 0 wday 3 yday 364
          dst nil gmtoff -28800 zone "PST")
  2> *1.(time-utc)
  0
  3> *1.(time-local)
  28800

This is also broken:

  1> (time-parse-utc "%H:%M:%z" "00:00:-0800")
  -28800

It must return 28800.

* time.c (time_meth): This function, which is the
imlpementation of the time-utc and time-local methods, must
subtract the gmtoff field, not add it. This is so that
a UTC time expressed in a local time zone will convert
back to the correct UTC epoch.
(time_parse_local, time_parse_utc): Here we likewise
must subtract the gmtoff.
---
 time.c | 8 ++++----
 txr.1  | 2 +-
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/time.c b/time.c
index b53fa37f..fbc1d42c 100644
--- a/time.c
+++ b/time.c
@@ -438,7 +438,7 @@ static val time_meth(val utc_p, val time_struct)
                                                 hour, min, sec, dst);
 
   if (gmtoff)
-    out = plus(out, gmtoff);
+    out = minus(out, gmtoff);
 
   return out;
 }
@@ -501,7 +501,7 @@ val time_parse_local(val format, val string)
 #if HAVE_TM_GMTOFF
   {
     long gmtoff = tms.TM_GMTOFF;
-    return num(mktime(&tms) + gmtoff);
+    return num(mktime(&tms) - gmtoff);
   }
 #else
   return num(mktime(&tms));
@@ -516,12 +516,12 @@ val time_parse_utc(val format, val string)
 #if HAVE_TIMEGM && HAVE_TM_GMTOFF
   {
     long gmtoff = tms.TM_GMTOFF;
-    return num_time(timegm(&tms) + gmtoff);
+    return num_time(timegm(&tms) - gmtoff);
   }
 #elif HAVE_TM_GMTOFF
   {
     long gmtoff = tms.TM_GMTOFF;
-    return num_time(timegm_hack(&tms) + tms.gmtoff);
+    return num_time(timegm_hack(&tms) - tms.gmtoff);
   }
 #elif HAVE_TIMEGM
   return num_time(timegm(&tms));
diff --git a/txr.1 b/txr.1
index 38172b93..f5a409b5 100644
--- a/txr.1
+++ b/txr.1
@@ -69054,7 +69054,7 @@ Note: if the
 .code gmtoff
 slot is not
 .codn nil ,
-its value is added to the returned result.
+its value is subtracted from the returned result.
 
 .coNP Method @ time-string
 .synb
-- 
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