On 05/10/2014 10:22, Paul Eggert wrote:
Trần Ngọc Quân wrote:

I find an other book about this topic:

Yes, there are a lot of books on Vietnamese dates and times!  All in Vietnamese, unfortunately for me.  The book by Trần Ngọc Quân appears to be one of the most authoritative.  Dershowitz and Reingold cites its 2005 edition in their book Calendrical Calculations (3rd ed.) <http://emr.cs.iit.edu/home/reingold/calendar-book/third-edition/>.  They also cite the 2000 edition of a calendrical book by L T Lân
Dr. Lê thành Lân
I find his email, but it I'm not sure he still use it. But I will try to contact.
lethanhlan@yahoo.de
and personal communications from Hồ Ngọc Đức -- none of which agree exactly, unfortunately.
I will try to contact Hồ Ngọc Đức.

> This is Hanoi, not Ho Chi Minh City.

We need to correct the Asia/Ho_Chi_Minh entry regardless of what we do about Hanoi, and that is what the patch does.  As I pointed out in my most recent email, it appears to be unnecessary to add an entry for Hanoi, since Hanoi does not have a unique time zone history since 1970.  In contrast, Ho Chi Minh City (now that its data are corrected) does have a unique time zone history since 1970 and therefore needs a distinct Zone.

I had planned to replace the Ho Chi Minh City zone with a link on the grounds that it was redundant, but this new information means we need to keep it.
I find other document about time zone in Vietnam (an archive on website). Note: date format is dd/mm/yyyy. It is  translated by google with some fix:

Previously, with the national stage or a part of the country using different time zones:

 - From 01/01/1943: Vietnam according to GMT + 8 (1 hour earlier than normal).
 - From 01/04/1945: Vietnam GMT + 9, 2 hours earlier than standard time.
 - From 01/04/1947, the French colonialists occupied GMT + 8.
 - In the South, from 01/07/1955 according to GMT + 7. To 01/01/1960, the time zone of southern GMT + 8.
 - In the north, according to Decision No. 121/CP signed on 08/08/1967 by Council of Government, from 08/08/1967 according to GMT + 7.
 - From 02/08/1976, the country standard time zone GMT + 7.

We seem have to find more document for both Vietnam and Lao, Cambodia. I hope I reply soon.


-- 
Trần Ngọc Quân.