Mark CET, CST6CDT etc. as obsolescent

These Zone names were present only for backward compatibility
with UNIX System V, last released in the 1990s.
Problem reported by Justin Grant in:
https://mm.icann.org/pipermail/tz/2024-May/058934.html
* Makefile (check_sorted): Do not check sorting of slashless
backzone names like CET since they’re in a special section.
(check_now): Also check for duplicate entries like CET vs
Europe/Brussels.
* NEWS: Mention the change.
* backward (CET, CST6CDT, EST, EST5EDT, MST, MST7MDT, HST, PST8PDT):
New backward-compatibility links.
(EUbackw): New ruleset, copied from EU in the ‘europe’ file.
(WET, MET, EET): Move here from ‘europe’, and use EUbackw.
* backzone (CET, EST, EST5EDT, CST6CDT, MST, MST7MDT, PST8PDT, HST):
Move here from ‘europe’ and ‘northamerica’.
* checknow.awk: Don’t assume zonenow.tab in diagnostic.
* europe (CET, WET, MET, EET): Move to ‘backward’
and make CET a link there.  The old CET is now in ‘backzone’.
* northamerica (EST, MST, HST, EST5EDT, CST6CDT, MST7MDT, PST8PDT):
Move to backzone.

Makefile

@@ -918,7 +918,7 @@ CHECK_CC_LIST = { n = split($$1,a,/,/); for (i=2; i<=n; i++) print a[1], a[i]; }
 check_sorted: backward backzone
 		$(AWK) '/^Link/ {printf "%.5d %s\n", g, $$3} !/./ {g++}' \
 		  backward | LC_ALL=C sort -cu
-		$(AWK) '/^Zone/ {print $$2}' backzone | LC_ALL=C sort -cu
+		$(AWK) '/^Zone.*\// {print $$2}' backzone | LC_ALL=C sort -cu
 		touch $@
 
 check_back:	checklinks.awk $(TDATA_TO_CHECK)
@@ -937,7 +937,7 @@ check_links:	checklinks.awk tzdata.zi
 # Check timestamps from now through 28 years from now, to make sure
 # that zonenow.tab contains all sequences of planned timestamps,
 # without any duplicate sequences.  In theory this might require
-# 2800 years but that would take a long time to check.
+# 2800+ years but that would take a long time to check.
 CHECK_NOW_TIMESTAMP = `./date +%s`
 CHECK_NOW_FUTURE_YEARS = 28
 CHECK_NOW_FUTURE_SECS = $(CHECK_NOW_FUTURE_YEARS) '*' 366 '*' 24 '*' 60 '*' 60
@@ -949,10 +949,20 @@ check_now:	checknow.awk date tzdata.zi zdump zic zone1970.tab zonenow.tab
 		  future=`expr $(CHECK_NOW_FUTURE_SECS) + $$now` && \
 		  ./zdump -i -t $$now,$$future \
 		     $$(find $$PWD/$@.dir/????*/ -type f) \
-		     >$@.dir/zdump.tab
+		     >$@.dir/zdump-now.tab && \
+		  ./zdump -i -t 0,$$future \
+		     $$(find $$PWD/$@.dir -name Etc -prune \
+			  -o -type f ! -name '*.tab' -print) \
+		     >$@.dir/zdump-1970.tab
 		$(AWK) \
-		  -v zdump_table=$@.dir/zdump.tab \
+		  -v zdump_table=$@.dir/zdump-now.tab \
 		  -f checknow.awk zonenow.tab
+		$(AWK) \
+		  'BEGIN {print "-\t-\tUTC"} /^Zone/ {print "-\t-\t" $$2}' \
+		  $(PRIMARY_YDATA) backward factory | \
+		 $(AWK) \
+		   -v zdump_table=$@.dir/zdump-1970.tab \
+		   -f checknow.awk
 		rm -fr $@.dir
 		touch $@
 

NEWS

@@ -3,12 +3,23 @@ News for the tz database
 Unreleased, experimental changes
 
   Briefly:
+    System V names are now obsolescent.
     The main data form now uses %z.
     The code now conforms to RFC 8536 for early timestamps.
     Improve historical data for Portugal and possessions.
 
   Changes to data
 
+    Names present only for compatibility with UNIX System V
+    (last released in the 1990s) have been moved to 'backward'.
+    These names, which for post-1970 timestamps mostly just duplicate
+    data of geographical names, were confusing downstream uses.
+    Names moved to 'backward' are now Links to geographical names if
+    there is an exact match since 1970, and remain Zones otherwise.
+    Those needing the previous TZDB behavior, which does not match any
+    real-world clocks, can find the old entries in 'backzone'.
+    (Problem reported by Justin Grant.)
+
     The main source files' time zone abbreviations now use %z,
     supported by zic since release 2015f and used in vanguard form
     since release 2022b.  For example, America/Sao_Paulo now contains

backward

@@ -1,9 +1,10 @@
-# tzdb links for backward compatibility
+# Links and zones for backward compatibility
 
 # This file is in the public domain, so clarified as of
 # 2009-05-17 by Arthur David Olson.
 
 # This file provides links from old or merged timezone names to current ones.
+# It also provides a few zone entries for old naming conventions.
 # Many names changed in 1993 and in 1995, and many merged names moved here
 # in the period from 2013 through 2022.  Several of these names are
 # also present in the file 'backzone', which has data important only
@@ -44,6 +45,8 @@ Link	America/Rio_Branco	Brazil/Acre	#= America/Porto_Acre
 Link	America/Noronha		Brazil/DeNoronha
 Link	America/Sao_Paulo	Brazil/East
 Link	America/Manaus		Brazil/West
+Link	Europe/Brussels		CET
+Link	America/Chicago		CST6CDT
 Link	America/Halifax		Canada/Atlantic
 Link	America/Winnipeg	Canada/Central
 # This line is commented out, as the name exceeded the 14-character limit
@@ -58,6 +61,8 @@ Link	America/Whitehorse	Canada/Yukon
 Link	America/Santiago	Chile/Continental
 Link	Pacific/Easter		Chile/EasterIsland
 Link	America/Havana		Cuba
+Link	America/Panama		EST
+Link	America/New_York	EST5EDT
 Link	Africa/Cairo		Egypt
 Link	Europe/Dublin		Eire
 # Vanguard section, for most .zi parsers.
@@ -96,6 +101,8 @@ Link	America/Jamaica		Jamaica
 Link	Asia/Tokyo		Japan
 Link	Pacific/Kwajalein	Kwajalein
 Link	Africa/Tripoli		Libya
+Link	America/Phoenix		MST
+Link	America/Denver		MST7MDT
 Link	America/Tijuana		Mexico/BajaNorte
 Link	America/Mazatlan	Mexico/BajaSur
 Link	America/Mexico_City	Mexico/General
@@ -314,5 +321,28 @@ Link	Europe/Kyiv		Europe/Kiev
 # Classically, Cyprus is in Asia; e.g. see Herodotus, Histories, I.72.
 # However, for various reasons many users expect to find it under Europe.
 Link	Asia/Nicosia		Europe/Nicosia
+Link	Pacific/Honolulu	HST
+Link	America/Los_Angeles	PST8PDT
 Link	Pacific/Guadalcanal	Pacific/Ponape	#= Pacific/Pohnpei
 Link	Pacific/Port_Moresby	Pacific/Truk	#= Pacific/Chuuk
+
+# These rules are stolen from the 'europe' file.
+# Rule	NAME	FROM	TO	-	IN	ON	AT	SAVE	LETTER/S
+Rule	EUbackw	1977	1980	-	Apr	Sun>=1	 1:00u	1:00	S
+Rule	EUbackw	1977	only	-	Sep	lastSun	 1:00u	0	-
+Rule	EUbackw	1978	only	-	Oct	 1	 1:00u	0	-
+Rule	EUbackw	1979	1995	-	Sep	lastSun	 1:00u	0	-
+Rule	EUbackw	1981	max	-	Mar	lastSun	 1:00u	1:00	S
+Rule	EUbackw	1996	max	-	Oct	lastSun	 1:00u	0	-
+
+# From Paul Eggert (2024-05-22):
+# The following zones pretend that standard time extends backward
+# indefinitely into the past, and so are ahistorical.
+# They are present for backward compatibility with older TZDB versions.
+# They are Zones (instead of being a Link like CET)
+# because they are not equivalent to any current Zone since 1970.
+
+# Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+Zone	WET		0:00	EUbackw	WE%sT
+Zone	MET		1:00	EUbackw	ME%sT
+Zone	EET		2:00	EUbackw	EE%sT

backzone

@@ -1829,6 +1829,24 @@ Zone	Pacific/Wake	11:06:28 -	LMT	1901
 Zone	Pacific/Wallis	12:15:20 -	LMT	1901
 			12:00	-	+12
 
+
+# From Paul Eggert (2024-05-22):
+# The following zones pretend that standard time extends backward
+# indefinitely into the past, and so are ahistorical.
+# In current TZDB these entries are links to geographical locations
+# that agree with the ahistorical zones since 1970.
+# These are in numeric rather than alphabetic order.
+
+# Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+Zone	HST		-10:00	-	HST
+Zone	PST8PDT		 -8:00	US	P%sT
+Zone	MST		 -7:00	-	MST
+Zone	MST7MDT		 -7:00	US	M%sT
+Zone	CST6CDT		 -6:00	US	C%sT
+Zone	EST		 -5:00	-	EST
+Zone	EST5EDT		 -5:00	US	E%sT
+Zone	CET		  1:00	C-Eur	CE%sT
+
 # Local Variables:
 # coding: utf-8
 # End:

checknow.awk

@@ -45,7 +45,7 @@ END {
  for (zone in zone_data) {
     data = zone_data[zone]
     if (!zonenow[data]) {
-      printf "zonenow.tab should have one of:%s\n", zones[data]
+      printf "Zone table should have one of:%s\n", zones[data]
       zonenow[data] = zone # This suppresses duplicate diagnostics.
       status = 1
     }

europe

@@ -730,14 +730,6 @@ Rule	Russia	1996	2010	-	Oct	lastSun	 2:00s	0	-
 # Take "abolishing daylight saving time" to mean that time is now considered
 # to be standard.
 
-# These are for backward compatibility with older versions.
-
-# Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
-Zone	WET		0:00	EU	WE%sT
-Zone	CET		1:00	C-Eur	CE%sT
-Zone	MET		1:00	C-Eur	ME%sT
-Zone	EET		2:00	EU	EE%sT
-
 # Previous editions of this database used abbreviations like MET DST
 # for Central European Summer Time, but this didn't agree with common usage.
 

northamerica

@@ -185,26 +185,6 @@ Rule	US	1987	2006	-	Apr	Sun>=1	2:00	1:00	D
 Rule	US	2007	max	-	Mar	Sun>=8	2:00	1:00	D
 Rule	US	2007	max	-	Nov	Sun>=1	2:00	0	S
 
-# From Arthur David Olson, 2005-12-19
-# We generate the files specified below to guard against old files with
-# obsolete information being left in the time zone binary directory.
-# We limit the list to names that have appeared in previous versions of
-# this time zone package.
-# We do these as separate Zones rather than as Links to avoid problems if
-# a particular place changes whether it observes DST.
-# We put these specifications here in the northamerica file both to
-# increase the chances that they'll actually get compiled and to
-# avoid the need to duplicate the US rules in another file.
-
-# Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
-Zone	EST		 -5:00	-	EST
-Zone	MST		 -7:00	-	MST
-Zone	HST		-10:00	-	HST
-Zone	EST5EDT		 -5:00	US	E%sT
-Zone	CST6CDT		 -6:00	US	C%sT
-Zone	MST7MDT		 -7:00	US	M%sT
-Zone	PST8PDT		 -8:00	US	P%sT
-
 # From U. S. Naval Observatory (1989-01-19):
 # USA  EASTERN       5 H  BEHIND UTC    NEW YORK, WASHINGTON
 # USA  EASTERN       4 H  BEHIND UTC    APR 3 - OCT 30