[tz] Add new timezone for Hanoi Capital, Vietnam

Tim Parenti tim at timtimeonline.com
Mon Feb 18 14:41:26 UTC 2019

On Mon, 18 Feb 2019 at 05:21, KP <khaiphan9x at gmail.com> wrote:

> Paul Eggert, I know that there must be some rules for forming time zones.

Those guidelines are well-established and are available in the theory.html
file of our distribution.  You can read the most recently-released version
of that file here: ftp://ftp.iana.org/tz/tzdb-2018i/theory.html

In particular, take note of these two guidelines:

   - "If all the clocks in a timezone have agreed since 1970, do not bother
   to include more than one timezone even if some of the clocks disagreed
   before 1970."
   - "Use the most populous among locations in a region, e.g., prefer
   Asia/Shanghai to Asia/Beijing."

It is these two guidelines, and this group's generally strong adherence to
them, that are the main reasons your request is meeting opposition.

> However, I see many exceptions such as Shanghai and Beijing, there are
> many other places that I have not mentioned.

As many have mentioned repeatedly, and as is demonstrated by example in the
guidelines quoted above, Asia/Beijing is NOT a zone that this database
provides, and that is NOT an exception that this group has made.  See these
lines in the most recently-released 'asia' file, which you can see at

# Beijing time, used throughout China; represented by Shanghai.
Zone Asia/Shanghai 8:05:43 - LMT 1901
8:00 Shang C%sT 1949 May 28
8:00 PRC C%sT

Although the comment line acknowledges that the official governmental name
of the time zone translates as "Beijing time", the actual name of the Zone
for tz purposes is Asia/Shanghai.  Both Shanghai and Beijing have always
had the same clock offsets since 1970 (in this case, UT+8 with some DST
from 1986 to 1991), so, by the first quoted guideline above, only one Zone
is needed in the tz database.  By the second quoted guideline
above, Shanghai was chosen because it has a larger population than Beijing.

If you are seeing Asia/Beijing as a time zone somewhere, then it is NOT
coming directly from the data this group provides.  You may find better
luck petitioning the downstream distributor of that data, as it is NOT a
valid example that will be found to be convincing here.

* * *

The case in Vietnam is indeed a little different, but it is already
covered.  Although clocks in Ho Chi Minh City and Hanoi HAVE differed since
1970 (by one hour until 1975-06-13), the clocks in Hanoi and Bangkok have
not differed (that is, they have both been at UT+7 the whole time since
1970).  So, by the first quoted guideline above, only one Zone is needed to
represent both areas.  By the second quoted guideline above, just like in
the China case, Bangkok was chosen because it has a larger population than

https://www.citypopulation.de/Thailand-Cities.html — Bangkok Metropolitan
https://www.citypopulation.de/Vietnam-Cities.html — Hà Nội including Hà Tây

By following the guidelines in theory.html, we find that Hanoi is already
considered properly covered by Asia/Bangkok for all timestamps since 1970,
and this is indeed already reflected by the inclusion of Vietnam's country
code in the corresponding entry in the zone1970.tab file, which indicates
that there are two options for Vietnam.  You can see that file at

TH,KH,LA,VN     +1345+10031     Asia/Bangkok    Indochina (most areas)
VN      +1045+10640     Asia/Ho_Chi_Minh        Vietnam (south)

Realistically, though, if you don't need timestamps from 1975 or earlier,
then even Asia/Ho_Chi_Minh will work for those purposes.

Tim Parenti

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