gmtime etc. now say "UTC", not "GMT"

POSIX is being revised to require this.
* NEWS, etcetera, theory.html: Mention this.
* date.c (dogmt): Simulate gmtime with TZ=UTC0, not GMT0.
* localtime.c (etc_utc): New constant.
(utc): Now an offset from etc_utc, replacing gmt.  All uses changed.
(gmtload): Load from "Etc/UTC", not from "GMT", so that
the abbreviation is "UTC" not "GMT".

NEWS

@@ -48,6 +48,9 @@ Unreleased, experimental changes
     zic now checks its input for NUL bytes and unterminated lines, and
     now supports input line lengths up to 2048 (not 512) bytes.
 
+    gmtime and related code now use the abbreviation "UTC" not "GMT".
+    POSIX is being revised to require this.
+
     When tzset and related functions set vestigial static variables
     like tzname, they now prefer specified timestamps to unspecified ones.
     (Problem reported by Almaz Mingaleev.)

date.c

@@ -120,7 +120,7 @@ dogmt(void)
 		register int	from;
 		register int	to;
 		register int	n;
-		static char	tzegmt0[] = "TZ=GMT0";
+		static char	tzeutc0[] = "TZ=UTC0";
 
 		for (n = 0;  environ[n] != NULL;  ++n)
 			continue;
@@ -131,7 +131,7 @@ dogmt(void)
 			exit(retval);
 		}
 		to = 0;
-		fakeenv[to++] = tzegmt0;
+		fakeenv[to++] = tzeutc0;
 		for (from = 1; environ[from] != NULL; ++from)
 			if (strncmp(environ[from], "TZ=", 3) != 0)
 				fakeenv[to++] = environ[from];

etcetera

@@ -17,12 +17,16 @@
 # behind GMT but uses the completely misleading abbreviation "GMT".
 
 Zone	Etc/GMT		0	-	GMT
+
+# The following zone is used by tzcode functions like gmtime,
+# which load the "UTC" file to handle seconds properly.
 Zone	Etc/UTC		0	-	UTC
 
 # The following link uses older naming conventions,
 # but it belongs here, not in the file 'backward',
-# as functions like gmtime load the "GMT" file to handle leap seconds properly.
-# We want this to work even on installations that omit the other older names.
+# as it is needed for tzcode releases through 2022a,
+# where functions like gmtime load "GMT" instead of the "Etc/UTC".
+# We want this to work even on installations that omit 'backward'.
 Link	Etc/GMT				GMT
 
 Link	Etc/UTC				Etc/Universal

localtime.c

@@ -74,7 +74,8 @@ static void unlock(void) { }
 
 static const char	wildabbr[] = WILDABBR;
 
-static const char	gmt[] = "GMT";
+static char const etc_utc[] = "Etc/UTC";
+static char const *utc = etc_utc + sizeof "Etc/" - 1;
 
 /*
 ** The DST rules to use if TZ has no rules and we can't load TZDEFRULES.
@@ -125,7 +126,7 @@ struct state {
 	time_t		ats[TZ_MAX_TIMES];
 	unsigned char	types[TZ_MAX_TIMES];
 	struct ttinfo	ttis[TZ_MAX_TYPES];
-	char chars[max(max(TZ_MAX_CHARS + CHARS_EXTRA, sizeof gmt),
+	char chars[max(max(TZ_MAX_CHARS + CHARS_EXTRA, sizeof "UTC"),
 		       2 * (MY_TZNAME_MAX + 1))];
 	struct lsinfo	lsis[TZ_MAX_LEAPS];
 
@@ -314,7 +315,7 @@ settzname(void)
 	int stddst_mask = 0;
 
 #if HAVE_TZNAME
-	tzname[0] = tzname[1] = (char *) (sp ? wildabbr : gmt);
+	tzname[0] = tzname[1] = (char *) (sp ? wildabbr : utc);
 	stddst_mask = 3;
 #endif
 #if USG_COMPAT
@@ -1393,8 +1394,8 @@ tzparse(const char *name, struct state *sp, struct state *basep)
 static void
 gmtload(struct state *const sp)
 {
-	if (tzload(gmt, sp, true) != 0)
-	  tzparse("GMT0", sp, NULL);
+	if (tzload(etc_utc, sp, true) != 0)
+	  tzparse("UTC0", sp, NULL);
 }
 
 /* Initialize *SP to a value appropriate for the TZ setting NAME.
@@ -1412,7 +1413,7 @@ zoneinit(struct state *sp, char const *name)
     sp->charcnt = 0;
     sp->goback = sp->goahead = false;
     init_ttinfo(&sp->ttis[0], 0, false, 0);
-    strcpy(sp->chars, gmt);
+    strcpy(sp->chars, utc);
     sp->defaulttype = 0;
     return 0;
   } else {
@@ -1666,7 +1667,7 @@ gmtsub(struct state const *sp, time_t const *timep, int_fast32_t offset,
 	** but this is no time for a treasure hunt.
 	*/
 	tmp->TM_ZONE = ((char *)
-			(offset ? wildabbr : gmtptr ? gmtptr->chars : gmt));
+			(offset ? wildabbr : gmtptr ? gmtptr->chars : utc));
 #endif /* defined TM_ZONE */
 	return result;
 }

theory.html

@@ -376,9 +376,11 @@ <h2 id="naming">Timezone identifiers</h2>
 on platforms that do not support POSIX-style <code>TZ</code> strings;
 no other source file other than <code>backward</code>
 contains links to its zones.
-One of <code>etcetera</code>'s names is <code>GMT</code>,
+One of <code>etcetera</code>'s names is <code>Etc/UTC</code>,
 used by functions like <code>gmtime</code> to obtain leap
 second information on platforms that support leap seconds.
+Another <code>etcetera</code> name, <code>GMT</code>,
+is used by older code releases.
 </p>
 </section>
 
@@ -468,6 +470,7 @@ <h2 id="abbreviations">Time zone abbreviations</h2>
       PST/PDT Philippine,
       SAST South Africa,
       SST Samoa,
+      UTC Universal,
       WAT/WAST West Africa,
       WET/WEST/WEMT Western European,
       WIB Waktu Indonesia Barat,
@@ -1295,7 +1298,8 @@ <h2 id="leapsec">Leap seconds</h2>
 if no time zone correction is desired,
 calls to <code>gmtime</code>-like functions
 also need to consult a <abbr>TZif</abbr> file,
-conventionally named <samp><abbr>GMT</abbr></samp>,
+conventionally named <samp><abbr>Etc/UTC</abbr></samp>
+(<samp><abbr>GMT</abbr></samp> in previous versions),
 to see whether leap second corrections are needed.
 To convert an application's <code>time_t</code> timestamps to or from
 POSIX <code>time_t</code> timestamps (for use when, say,