diff --git a/Makefile b/Makefile
index 8714c16..ce018ec 100644
--- a/Makefile
+++ b/Makefile
@@ -529,7 +529,7 @@ TZS_YEAR= 2050
TZS_CUTOFF_FLAG= -c $(TZS_YEAR)
TZS= to$(TZS_YEAR).tzs
TZS_NEW= to$(TZS_YEAR)new.tzs
-TZS_DEPS= $(PRIMARY_YDATA) asctime.c localtime.c \
+TZS_DEPS= $(YDATA) asctime.c localtime.c \
private.h tzfile.h zdump.c zic.c
# EIGHT_YARDS is just a yard short of the whole ENCHILADA.
EIGHT_YARDS = $(COMMON) $(DOCS) $(SOURCES) $(DATA) $(MISC) tzdata.zi
diff --git a/NEWS b/NEWS
index 6d1cd09..435a40f 100644
--- a/NEWS
+++ b/NEWS
@@ -26,6 +26,23 @@ Unreleased, experimental changes
(Thanks to P Chan, Michael Deckers, Alexander Krivenyshev and
Alois Treindl.)
+ Merge timezones whose timestamps agree since 1970, since pre-1970
+ timestamps are out of scope. This change does not affect
+ post-1970 timestamps, and timezone historians who build with 'make
+ PACKRATDATA=backzone' should see no changes to pre-1970 timestamps.
+ When merging, keep the most-populous location's data, and move
+ data for other locations to 'backzone' with a backward
+ compatibility link in 'backward'. Also, merge timezones with
+ neither post-1970 clock changes nor alphabetic abbreviations into
+ their Etc counterparts. For example, move the Europe/Oslo data to
+ 'backzone' with a link in 'backward' from Europe/Berlin because
+ the two timezones' timestamps agree since 1970; this change
+ affects some pre-1966 timestamps in Europe/Oslo because Berlin and
+ Oslo disagreed before 1966. For another example, move the
+ Asia/Aden data to 'backzone' with a link in 'backward' from
+ Etc/GMT-3. Affected entries range from Africa/Abidjan to
+ Pacific/Yap.
+
Changes to maintenance procedure
The new file SECURITY covers how to report security-related bugs.
diff --git a/backzone b/backzone
index 6c77378..b58f4d8 100644
--- a/backzone
+++ b/backzone
@@ -619,6 +619,8 @@ Zone America/Creston -7:46:04 - LMT 1884
Zone America/Curacao -4:35:47 - LMT 1912 Feb 12 # Willemstad
-4:30 - -0430 1965
-4:00 - AST
+Link America/Curacao America/Kralendijk
+Link America/Curacao America/Lower_Princes
# Dominica
Zone America/Dominica -4:05:36 - LMT 1911 Jul 1 0:01 # Roseau
@@ -763,6 +765,9 @@ Zone America/Nassau -5:09:30 - LMT 1912 Mar 2
# Trinidad and Tobago
Zone America/Port_of_Spain -4:06:04 - LMT 1912 Mar 2
-4:00 - AST
+Link America/Port_of_Spain America/Marigot
+Link America/Port_of_Spain America/St_Barthelemy
+Link America/Port_of_Spain America/Virgin
# Argentina
# This entry was intended for the following areas, but has been superseded by
@@ -1228,6 +1233,7 @@ Rule Iceland 1967 only - Oct 29 1:00s 0 -
Zone Atlantic/Reykjavik -1:28 - LMT 1908
-1:00 Iceland -01/+00 1968 Apr 7 1:00s
0:00 - GMT
+Link Atlantic/Reykjavik Iceland
# South Georgia
Zone Atlantic/South_Georgia -2:26:08 - LMT 1890 # Grytviken
@@ -1520,6 +1526,7 @@ Zone Europe/Oslo 0:43:00 - LMT 1895 Jan 1
1:00 C-Eur CE%sT 1945 Apr 2 2:00
1:00 Norway CE%sT 1980
1:00 EU CE%sT
+Link Europe/Oslo Arctic/Longyearbyen
# Bosnia and Herzegovina
Zone Europe/Sarajevo 1:13:40 - LMT 1884
@@ -1708,6 +1715,8 @@ Zone Pacific/Chuuk -13:52:52 - LMT 1844 Dec 31
10:00 - +10 1941 Apr 1
9:00 - +09 1945 Aug
10:00 - +10
+Link Pacific/Chuuk Pacific/Truk
+Link Pacific/Chuuk Pacific/Yap
# Tuvalu
Zone Pacific/Funafuti 11:56:52 - LMT 1901
@@ -1767,6 +1776,7 @@ Zone Pacific/Pohnpei -13:27:08 - LMT 1844 Dec 31 # Kolonia
10:00 - +10 1941 Apr 1
9:00 - +09 1945 Aug
11:00 - +11
+Link Pacific/Pohnpei Pacific/Ponape
# Papua New Guinea
Zone Pacific/Port_Moresby 9:48:40 - LMT 1880
diff --git a/etcetera b/etcetera
index 1dc7411..dc21c4d 100644
--- a/etcetera
+++ b/etcetera
@@ -12,6 +12,8 @@
# Starting with POSIX 1003.1-2001, the entries below are all
# unnecessary as settings for the TZ environment variable. E.g.,
# instead of TZ='Etc/GMT+4' one can use the POSIX setting TZ='<-04>+4'.
+# However, some entries are needed if 'backward' is used:
+# for example, the 'backward' file links Etc/GMT to Africa/Abidjan.
#
# Do not use a POSIX TZ setting like TZ='GMT+4', which is four hours
# behind GMT but uses the completely misleading abbreviation "GMT".
diff --git a/theory.html b/theory.html
index 5251c96..47a7b2d 100644
--- a/theory.html
+++ b/theory.html
@@ -42,12 +42,13 @@ href="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch
(1970-01-01 00:00:00 UTC).
-The database labels each timezone with a notable location and
-records all known clock transitions for that location.
Although 1970 is a somewhat-arbitrary cutoff, there are significant
challenges to moving the cutoff earlier even by a decade or two, due
to the wide variety of local practices before computer timekeeping
became prevalent.
+Most timezones correspond to a notable location and the database
+records all known clock transitions for that location;
+some timezones correspond instead to a fixed UTC offset.
@@ -68,7 +70,7 @@ time but differs from other timezones for some timestamps after 1970.
-Clock transitions before 1970 are recorded for each timezone,
+Clock transitions before 1970 are recorded for location-based timezones,
because most systems support timestamps before 1970 and could
misbehave if data entries were omitted for pre-1970 transitions.
However, the database is not designed for and does not suffice for