Many corrections to data for Vietnam, 1906-1975.

* asia (Asia/Ho_Chi_Minh):
Rewrite the data from 1906 through 1975, using new information
from Trần Tiến Bình's book.  (Thanks to Trần Ngọc Quân.)
New abbreviation IDT for UT+8 in Indochina.
* checktab.awk (tztab): Remove special cases for this zone.
* zone1970.tab: Add Asia/Ho_Chi_Minh, and add Asia/Bangkok commentary.
* NEWS: Document this.

NEWS

@@ -4,6 +4,14 @@ Unreleased, experimental changes
 
   Changes affecting past time stamps
 
+    Many time stamps have been corrected for Asia/Ho_Chi_Minh before 1976
+    (thanks to Trần Ngọc Quân for an indirect pointer to Trần Tiến Bình's
+    authoritative book).  Asia/Ho_Chi_Minh has been added to
+    zone1970.tab, to give tzselect users in Vietnam two choices,
+    since north and south Vietnam disagreed after our 1970 cutoff.
+    The new abbreviation IDT stands for the pre-1976 use of UT+8 in
+    Indochina, to distinguish it better from ICT (UT+7).
+
     Asia/Phnom_Penh and Asia/Vientiane have been turned into links, as
     they differed from existing zones only for older time stamps.  As
     usual, these changes affect pre-1970 time stamps only.  Their old

asia

@@ -47,10 +47,11 @@
 #	3:30 IRST IRDT	Iran
 #	4:00 GST	Gulf*
 #	5:30 IST	India
-#	7:00 ICT	Indochina*
+#	7:00 ICT	Indochina, most times and locations*
 #	7:00 WIB	west Indonesia (Waktu Indonesia Barat)
 #	8:00 WITA	central Indonesia (Waktu Indonesia Tengah)
 #	8:00 CST	China
+#	8:00 IDT	Indochina, 1943-45, 1947-55, 1960-75 (some locations)*
 #	8:00 JWST	Western Standard Time (Japan, 1896/1937)*
 #	9:00 JCST	Central Standard Time (Japan, 1896/1937)
 #	9:00 WIT	east Indonesia (Waktu Indonesia Timur)
@@ -2763,22 +2764,56 @@ Zone	Asia/Tashkent	4:37:11 -	LMT	1924 May  2
 
 # Vietnam
 
-# From Paul Eggert (2013-02-21):
+# From Paul Eggert (2014-10-04):
 # Milne gives 7:16:56 for the meridian of Saigon in 1899, as being
 # used in Lower Laos, Cambodia, and Annam.  But this is quite a ways
 # from Saigon's location.  For now, ignore this and stick with Shanks
-# and Pottenger.
+# and Pottenger for LMT before 1906.
 
 # From Arthur David Olson (2008-03-18):
 # The English-language name of Vietnam's most populous city is "Ho Chi Minh
 # City"; use Ho_Chi_Minh below to avoid a name of more than 14 characters.
 
-# From Shanks & Pottenger:
+# From Paul Eggert (2014-10-04) after a heads-up from Trần Ngọc Quân:
+# http://www.thoigian.com.vn/?mPage=P80D01
+# lists helpful info, translated by Brian Inglis in:
+# http://mm.icann.org/pipermail/tz/2014-October/021654.html
+# The info cites an authoritative book by Trần Tiến Bình; a 2014 edition is
+# "Lịch Việt Nam: thế kỉ XX-XXI (1901-2100) & niên biểu lịch sử Việt Nam"
+# http://bookaholic.vn/lich-viet-nam-the-ki-xx-xxi-1901-2100-va-bien-bieu-lich-su-viet-nam/
+# I have asked my library for its copy of the book (a 2005 edition)
+# and plan to check that.  In the meantime the summary in Thời gian is much
+# better than Shanks, and is the basis for the following info.
+#
+# The 1906 transition was effective July 1 and standardized Indochina to
+# Phù Liễn Observatory, legally 104 deg. 17'17" east of Paris.
+# It's unclear whether this meant legal Paris Mean Time (00:09:21) or
+# the Paris Meridian (2 deg. 20'14.03" E); the former yields 07:06:30.1333...
+# and the latter 07:06:29.333... so either way it rounds to 07:06:30,
+# which is used below even though the modern-day Phù Liễn Observatory
+# is closer to 07:06:31.  Abbreviate Phù Liễn Mean Time as PLMT.
+#
+# The following transitions occurred in Indochina in general (before 1954)
+# and in South Vietnam in particular (after 1954):
+# To 07:00 on 1911-05-01.
+# To 08:00 on 1942-12-31 at 23:00.
+# To 09:00 in 1945-03-14 at 23:00.
+# To 07:00 on 1945-09-02 in Vietnam.
+# To 08:00 on 1947-04-01 in French-controlled Indochina.
+# To 07:00 on 1955-07-01 in South Vietnam.
+# To 08:00 on 1959-12-31 at 23:00 in South Vietnam.
+# To 07:00 on 1975-06-13 in South Vietnam.
+
 # Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
-Zone	Asia/Ho_Chi_Minh	7:06:40 -	LMT	1906 Jun  9
+Zone Asia/Ho_Chi_Minh	7:06:40 -	LMT	1906 Jul  1
-			7:06:20	-	SMT	1911 Mar 11  0:01 # Saigon MT?
+			7:06:30	-	PLMT	1911 May  1
-			7:00	-	ICT	1912 May
+			7:00	-	ICT	1942 Dec 31 23:00
-			8:00	-	ICT	1931 May
+			8:00	-	IDT	1945 Mar 14 23:00
+			9:00	-	JST	1945 Sep  2
+			7:00	-	ICT	1947 Apr  1
+			8:00	-	IDT	1955 Jul  1
+			7:00	-	ICT	1959 Dec 31 23:00
+			8:00	-	IDT	1975 Jun 13
 			7:00	-	ICT
 
 # Yemen

checktab.awk

@@ -25,7 +25,6 @@ BEGIN {
 	  tztab["America/Cayman"] = 1
 	  tztab["Asia/Aden"] = 1
 	  tztab["Asia/Bahrain"] = 1
-	  tztab["Asia/Ho_Chi_Minh"] = 1
 	  tztab["Asia/Kuwait"] = 1
 	  tztab["Asia/Muscat"] = 1
 	  tztab["Indian/Antananarivo"] = 1

zone1970.tab

@@ -315,7 +315,7 @@ SY	+3330+03618	Asia/Damascus
 TC	+2128-07108	America/Grand_Turk
 TD	+1207+01503	Africa/Ndjamena
 TF	-492110+0701303	Indian/Kerguelen	Kerguelen, St Paul I, Amsterdam I
-TH,KH,LA,VN	+1345+10031	Asia/Bangkok
+TH,KH,LA,VN	+1345+10031	Asia/Bangkok	most of Indochina
 TJ	+3835+06848	Asia/Dushanbe
 TK	-0922-17114	Pacific/Fakaofo
 TL	-0833+12535	Asia/Dili
@@ -363,6 +363,7 @@ UY	-3453-05611	America/Montevideo
 UZ	+3940+06648	Asia/Samarkand	west Uzbekistan
 UZ	+4120+06918	Asia/Tashkent	east Uzbekistan
 VE	+1030-06656	America/Caracas
+VN	+1045+10640	Asia/Ho_Chi_Minh	south Vietnam
 VU	-1740+16825	Pacific/Efate
 WF	-1318-17610	Pacific/Wallis
 WS	-1350-17144	Pacific/Apia