Tim Parenti wrote:

... ICT referring to different UT offsets for several periods.  Perhaps, prior to
reunification, NICT and SICT should be used for "North/South Indochina Time"?

It wasn't North vs South.  The new data say that French Indochina mostly used UT+7, but sometimes all of French Indochina used UT+8, sometimes just South Vietnam, sometimes more of a hodgepodge.  I had independently run into this problem and thought of using the abbreviation "IDT" (short for InDochina Time) for UT+8 in Indochina to help clear this up.

There's a bigger problem, though.  The new data also tell us that our entries for Laos and Cambodia are almost entirely bogus.  We already knew that, and I had slated them for 'backzone' anyway, so now's a good time to do that.  As a corollary, although we clearly need to fix Asia/Ho_Chi_Minh, we don't need a separate Zone for Asia/Hanoi as it is identical to Asia/Bangkok since 1970. This will give us a simpler fix and an easier-to-maintain result.

A proposed pair of patches is attached.  The first moves the bogus data for Cambodia and Laos to 'backzone'.  The second fixes Asia/Ho_Chi_Minh and adds a line to zone1970.tab so that users in Vietnam now have two choices, one for each half of the country.

+Zone Asia/Ho_Chi_Minh	7:06:40 -	LMT	1906 Jul  1
+			7:06:30	-	PLMT	1911 May  1
+			7:00	-	ICT	1942 Dec 31 23:00
+			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

This is Hanoi, not Ho Chi Minh City.

I find an other book about this topic:

Name: Lịch âm dương Việt Nam 1900-2010
Author: Nguyễn Văn Chung
Publisher: Nhà xuất bản văn hóa dân tộc
Edition: 2001

-- 
Trần Ngọc Quân.