Move out-of-scope UTC+3 African zones to 'backzone'.

* africa (Africa/Addis_Ababa, Africa/Asmara, Africa/Dar_es_Salaam)
(Africa/Djibouti, Africa/Kampala, Africa/Mogadishu, Indian/Antananarivo)
(Indian/Comoro, Indian/Mayotte):
Change these from zones to links to Africa/Nairobi.
* backzone: Move the old data here.
* checktab.awk (tztab): Remove special cases for these zones.
* NEWS: Document the above.

NEWS

@@ -10,6 +10,15 @@ Unreleased, experimental changes
     longer guess that Pyongyang mimicked Seoul time after World War II,
     as this is politically implausible.
 
+    Some more zones have been turned into links, when they differed
+    from existing zones only for older time stamps.  As usual,
+    these changes affect UTC offsets in pre-1970 time stamps only.
+    Their old contents have been moved to the 'backzone' file.
+    The affected zones are: Africa/Addis_Ababa, Africa/Asmara,
+    Africa/Dar_es_Salaam, Africa/Djibouti, Africa/Kampala,
+    Africa/Mogadishu, Indian/Antananarivo, Indian/Comoro, and
+    Indian/Mayotte.
+
   Changes affecting commentary
 
     The commentary is less enthusiastic about Shanks as a source,

africa

@@ -64,7 +64,6 @@
 #	 3:00	CAST	Central Africa Summer Time (no longer used)
 #	 3:00	SAST	South Africa Summer Time (no longer used)
 #	 3:00	EAT	East Africa Time
-#	 4:00	EAST	East Africa Summer Time (no longer used)
 
 # Algeria
 # Rule	NAME	FROM	TO	TYPE	IN	ON	AT	SAVE	LETTER/S
@@ -145,9 +144,7 @@ Zone	Africa/Ndjamena	1:00:12 -	LMT	1912        # N'Djamena
 			1:00	-	WAT
 
 # Comoros
-# Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
-Zone	Indian/Comoro	2:53:04 -	LMT	1911 Jul # Moroni, Gran Comoro
-			3:00	-	EAT
+# See Africa/Nairobi.
 
 # Democratic Republic of the Congo
 # See Africa/Lagos for the western part and Africa/Maputo for the eastern.
@@ -171,9 +168,7 @@ Link Africa/Abidjan Africa/Sao_Tome	# São Tomé and Príncipe
 Link Africa/Abidjan Atlantic/St_Helena	# St Helena
 
 # Djibouti
-# Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
-Zone	Africa/Djibouti	2:52:36 -	LMT	1911 Jul
-			3:00	-	EAT
+# See Africa/Nairobi.
 
 ###############################################################################
 
@@ -386,27 +381,8 @@ Zone	Africa/Cairo	2:05:09 -	LMT	1900 Oct
 # See Africa/Lagos.
 
 # Eritrea
-# Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
-Zone	Africa/Asmara	2:35:32 -	LMT	1870
-			2:35:32	-	AMT	1890        # Asmara Mean Time
-			2:35:20	-	ADMT	1936 May  5 # Adis Dera MT
-			3:00	-	EAT
-
 # Ethiopia
-# From Paul Eggert (2014-07-31):
-# Like the Swahili of Kenya and Tanzania, many Ethiopians keep a
-# 12-hour clock starting at our 06:00, so their "8 o'clock" is our
-# 02:00 or 14:00.  Keep this in mind when you ask the time in Amharic.
-#
-# Shanks & Pottenger write that Ethiopia had six narrowly-spaced time
-# zones between 1870 and 1890, that they merged to 38E50 (2:35:20) in
-# 1890, and that they switched to 3:00 on 1936-05-05.  Perhaps 38E50
-# was for Adis Dera.  Quite likely the Shanks data entries are wrong
-# anyway.
-# Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
-Zone Africa/Addis_Ababa	2:34:48 -	LMT	1870
-			2:35:20	-	ADMT	1936 May  5 # Adis Dera MT
-			3:00	-	EAT
+# See Africa/Nairobi.
 
 # Gabon
 # See Africa/Lagos.
@@ -450,6 +426,15 @@ Zone	Africa/Nairobi	2:27:16	-	LMT	1928 Jul
 			2:30	-	BEAT	1940
 			2:45	-	BEAUT	1960
 			3:00	-	EAT
+Link Africa/Nairobi Africa/Addis_Ababa	 # Ethiopia
+Link Africa/Nairobi Africa/Asmara	 # Eritrea
+Link Africa/Nairobi Africa/Dar_es_Salaam # Tanzania
+Link Africa/Nairobi Africa/Djibouti
+Link Africa/Nairobi Africa/Kampala	 # Uganda
+Link Africa/Nairobi Africa/Mogadishu	 # Somalia
+Link Africa/Nairobi Indian/Antananarivo	 # Madagascar
+Link Africa/Nairobi Indian/Comoro
+Link Africa/Nairobi Indian/Mayotte
 
 # Lesotho
 # See Africa/Johannesburg.
@@ -527,11 +512,7 @@ Zone	Africa/Tripoli	0:52:44 -	LMT	1920
 			2:00	-	EET
 
 # Madagascar
-# Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
-Zone Indian/Antananarivo 3:10:04 -	LMT	1911 Jul
-			3:00	-	EAT	1954 Feb 27 23:00s
-			3:00	1:00	EAST	1954 May 29 23:00s
-			3:00	-	EAT
+# See Africa/Nairobi.
 
 # Malawi
 # See Africa/Maputo.
@@ -634,9 +615,7 @@ Zone Indian/Mauritius	3:50:00 -	LMT	1907 # Port Louis
 # no information; probably like Indian/Mauritius
 
 # Mayotte
-# Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
-Zone	Indian/Mayotte	3:00:56 -	LMT	1911 Jul # Mamoutzou
-			3:00	-	EAT
+# See Africa/Nairobi.
 
 # Morocco
 # See the 'europe' file for Spanish Morocco (Africa/Ceuta).
@@ -1048,11 +1027,7 @@ Zone	Indian/Mahe	3:41:48 -	LMT	1906 Jun # Victoria
 # See Africa/Abidjan.
 
 # Somalia
-# Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
-Zone Africa/Mogadishu	3:01:28 -	LMT	1893 Nov
-			3:00	-	EAT	1931
-			2:30	-	BEAT	1957
-			3:00	-	EAT
+# See Africa/Nairobi.
 
 # South Africa
 # Rule	NAME	FROM	TO	TYPE	IN	ON	AT	SAVE	LETTER/S
@@ -1095,11 +1070,7 @@ Link Africa/Khartoum Africa/Juba
 # See Africa/Johannesburg.
 
 # Tanzania
-# Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
-Zone Africa/Dar_es_Salaam 2:37:08 -	LMT	1931
-			3:00	-	EAT	1948
-			2:45	-	BEAUT	1961
-			3:00	-	EAT
+# See Africa/Nairobi.
 
 # Togo
 # See Africa/Abidjan.
@@ -1205,12 +1176,7 @@ Zone	Africa/Tunis	0:40:44 -	LMT	1881 May 12
 			1:00	Tunisia	CE%sT
 
 # Uganda
-# Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
-Zone	Africa/Kampala	2:09:40 -	LMT	1928 Jul
-			3:00	-	EAT	1930
-			2:30	-	BEAT	1948
-			2:45	-	BEAUT	1957
-			3:00	-	EAT
+# See Africa/Nairobi.
 
 # Zambia
 # Zimbabwe

backzone

@@ -40,6 +40,27 @@
 # As explained in the zic man page, the zone columns are:
 # Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
 
+# Ethiopia
+# From Paul Eggert (2014-07-31):
+# Like the Swahili of Kenya and Tanzania, many Ethiopians keep a
+# 12-hour clock starting at our 06:00, so their "8 o'clock" is our
+# 02:00 or 14:00.  Keep this in mind when you ask the time in Amharic.
+#
+# Shanks & Pottenger write that Ethiopia had six narrowly-spaced time
+# zones between 1870 and 1890, that they merged to 38E50 (2:35:20) in
+# 1890, and that they switched to 3:00 on 1936-05-05.  Perhaps 38E50
+# was for Adis Dera.  Quite likely the Shanks data entries are wrong
+# anyway.
+Zone Africa/Addis_Ababa	2:34:48 -	LMT	1870
+			2:35:20	-	ADMT	1936 May  5 # Adis Dera MT
+			3:00	-	EAT
+
+# Eritrea
+Zone	Africa/Asmara	2:35:32 -	LMT	1870
+			2:35:32	-	AMT	1890        # Asmara Mean Time
+			2:35:20	-	ADMT	1936 May  5 # Adis Dera MT
+			3:00	-	EAT
+
 # Mali (southern)
 Zone	Africa/Bamako	-0:32:00 -	LMT	1912
 			 0:00	-	GMT	1934 Feb 26
@@ -79,6 +100,16 @@ Zone	Africa/Dakar	-1:09:44 -	LMT	1912
 			-1:00	-	WAT	1941 Jun
 			 0:00	-	GMT
 
+# Tanzania
+Zone Africa/Dar_es_Salaam 2:37:08 -	LMT	1931
+			3:00	-	EAT	1948
+			2:45	-	BEAUT	1961
+			3:00	-	EAT
+
+# Djibouti
+Zone	Africa/Djibouti	2:52:36 -	LMT	1911 Jul
+			3:00	-	EAT
+
 # Cameroon
 # Whitman says they switched to 1:00 in 1920; go with Shanks & Pottenger.
 Zone	Africa/Douala	0:38:48	-	LMT	1912
@@ -118,6 +149,13 @@ Zone	Africa/Juba	2:06:24 -	LMT	1931
 			2:00	Sudan	CA%sT	2000 Jan 15 12:00
 			3:00	-	EAT
 
+# Uganda
+Zone	Africa/Kampala	2:09:40 -	LMT	1928 Jul
+			3:00	-	EAT	1930
+			2:30	-	BEAT	1948
+			2:45	-	BEAUT	1957
+			3:00	-	EAT
+
 # Rwanda
 Zone	Africa/Kigali	2:00:16 -	LMT	1935 Jun
 			2:00	-	CAT
@@ -174,6 +212,12 @@ Zone	Africa/Maseru	1:50:00 -	LMT	1903 Mar
 Zone	Africa/Mbabane	2:04:24 -	LMT	1903 Mar
 			2:00	-	SAST
 
+# Somalia
+Zone Africa/Mogadishu	3:01:28 -	LMT	1893 Nov
+			3:00	-	EAT	1931
+			2:30	-	BEAT	1957
+			3:00	-	EAT
+
 # Niger
 Zone	Africa/Niamey	 0:08:28 -	LMT	1912
 			-1:00	-	WAT	1934 Feb 26
@@ -517,5 +561,19 @@ Zone	Europe/Zagreb	1:03:52	-	LMT	1884
 			1:00	-	CET	1982 Nov 27
 			1:00	EU	CE%sT
 
+# Madagascar
+Zone Indian/Antananarivo 3:10:04 -	LMT	1911 Jul
+			3:00	-	EAT	1954 Feb 27 23:00s
+			3:00	1:00	EAST	1954 May 29 23:00s
+			3:00	-	EAT
+
+# Comoros
+Zone	Indian/Comoro	2:53:04 -	LMT	1911 Jul # Moroni, Gran Comoro
+			3:00	-	EAT
+
+# Mayotte
+Zone	Indian/Mayotte	3:00:56 -	LMT	1911 Jul # Mamoutzou
+			3:00	-	EAT
+
 # US minor outlying islands
 Zone Pacific/Johnston	-10:00	-	HST

checktab.awk

@@ -15,21 +15,12 @@ BEGIN {
 	# Some more special cases; these are zones that should probably
 	# be turned into links.
 	if (zone_table == "zone1970.tab") {
-	  tztab["Africa/Addis_Ababa"] = 1
-	  tztab["Africa/Asmara"] = 1
-	  tztab["Africa/Dar_es_Salaam"] = 1
-	  tztab["Africa/Djibouti"] = 1
-	  tztab["Africa/Kampala"] = 1
-	  tztab["Africa/Mogadishu"] = 1
 	  tztab["America/Antigua"] = 1
 	  tztab["America/Cayman"] = 1
 	  tztab["Asia/Aden"] = 1
 	  tztab["Asia/Bahrain"] = 1
 	  tztab["Asia/Kuwait"] = 1
 	  tztab["Asia/Muscat"] = 1
-	  tztab["Indian/Antananarivo"] = 1
-	  tztab["Indian/Comoro"] = 1
-	  tztab["Indian/Mayotte"] = 1
 	  tztab["Pacific/Midway"] = 1
 	  tztab["Pacific/Saipan"] = 1
 	}