Vietnam timezone history probably wrong

[Alois Treindl]

tzdata gives for Vietnam timezone
# From Shanks & Pottenger:
# Zone  NAME            GMTOFF  RULES   FORMAT  [UNTIL]
Zone    Asia/Ho_Chi_Minh        7:06:40 -       LMT     1906 Jun  9
                         7:06:20 -       SMT     1911 Mar 11 0:01 # 
Saigon MT?
                         7:00    -       ICT     1912 May
                         8:00    -       ICT     1931 May
                         7:00    -       ICT

This is possibly wrong for the years beween 1911 and 1976.
I am us9ing the source
Gabriel, Traite de l'heure dans le monde, edition 1991.

It deals with Vietnam under the entry 'Indochine Française' until 1954, 
and then separately with North and South Vietnam, until the unification 
in 1976.

Gabriel gives this timezone information:
French Indochina (including Vietnam, Cambodia, Laos)
until 1 July 1906: local mean time
until 1 May 1911: 7h6m44s east (meridian of Saigon)
until 1 Jan 1943: 7h east
from 1 Jan 1943: 8h east
(with Daylight saving time, i.e. 9h east, between 14 March and 1 Sept 1945)

North Vietnam
until 15 April 1955: 8h east
until 1 Jan 1960: 7h east
until 1976 : 8h east
For 1976, Gabriel does not give a transition date. The official 
reunification was on 2 July 1976.
since 1976: 7h east

South Vietnam:
until 1976 (for date, see above): 8h east
since 1976: 7h east

I have searched for corroboration of the fact that Vietnam was on GMT + 
8h in years when tzdata and Shanks claim 7h.

For siginificant timed events during the Vietnam war, there must be 
newspaper reports which give local Vietnam time and corresponding US time.

One such report I have found is the last helicopter out of Saigon, 
according to TIME magazine.
http://www.time.com/time/magazine/article/0,9171,917411-2,00.html

It says:
Finally, at 5 p.m. Washington tune—it was then 5 a.m. in 
Saigon—Kissinger told the President that Martin was closing down the 
embassy and destroying its communications equipment. Minutes later, a 
helicopter broadcast the message: "Lady Ace Zero Nine, Code Two is 
aboard." Lady Ace 09 was the chopper's own call signal; Code Two 
designates an ambassador.

This is 5 PM on 29 Paril 1975. Washington DC was on DST, i.e. on 4h west 
timezone. Universal time is 21:00  (09 pm).

In Vietnam it is already the 30th of April, 5 am, in the morning. This 
means that Vietnam was 8 hours ahead of Universal time!

This means that, at least for April 1975, the source Gabriel is right 
and Shanks and tzdata are wrong, because they claim 7h east.

Conclusion: it is evident that Vietnam timezone history needs 
corrections, also in tzdata, because it should be correct at least 
starting from 1970.

I would be reluctant to simply copy the Gabriel source, as it fails to 
give a transition date in 1976. But research is needed. The best would 
be local Newspaper archives in Saigon or Hanoi, if such exist.



-- 
|| Alois Treindl,  Astrodienst AG,  mailto:alois at astro.com
|| Zollikon/Zurich, Switzerland
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