On Feb 18, 2019, at 9:07 AM, KP <khaiphan9x@gmail.com> wrote:

> It is possible to link Hanoi to Asia/Ho_Chi_Minh instead of Asia/Bangkok

...but doing so means that pre-time-zone-unification but post-1970-01-01 00:00:00 UTC dates will not be converted correctly, so it would be incorrect to do so.

> there is no document saying that North Vietnam accepted Asia/Bangkok as a representative time zone.

What matters isn't whether they officially accepted it; what matters is whether they, in practice, kept the same time.

> However, I did not change the idea that Hanoi is the best representative for Vietnam,

Given that there are, apparently, post-1970-01-01 00:00:00 UTC dates/times where local time differed between locations in what was then North Vietnam and what was then South Vietnam, there isn't a single tzdb region that represents Vietnam, there are two.