Why is Bangkok's time taken from 1920, but Hanoi's time is only taken from 1970?

Vào 2:19, Th 3, 19 thg 2, 2019 Guy Harris <guy@alum.mit.edu đã viết:

On Feb 18, 2019, at 10:22 AM, KP <khaiphan9x@gmail.com> wrote:

> Timezones used in Vietnam are +7:00, +8:00, +9:00 instead of +7:00 only like Bangkok, Thailand

...prior to 1970-01-01 00:00:00. That's why Asia/Hanoi is in backzone rather than asia.