[tz] North Vietnam 1970 to 1975 - Asia/Hanoi belongs into main file

[Paul Eggert]

On 6/28/20 7:42 AM, Alois Treindl wrote:
> The public geonames.org database  assigns the zone Asia/Ho_Chi_Minh for all
> location in country VN (Vietnam).
> This indicates that the comment above is not followed.

That's a problem with the geonames.org database, and I suggest reporting the bug
to them. The bug should be reported regardless of whether we change tzdb since
geonames.org should work with what we have now as well as with what we might
have in the future. You can point them at the zone1970.tab file to help explain
the situation.

A little background here. We don't have the resources to create entries for
every location in this part of southeast Asia, with different transition times
for when locations changed hands between North and South Vietnam during the
1970-1975 conflict. So we approximate by using Ho Chi Minh as the representative
for clocks in south Vietnam, and Bangkok as the representative for clocks in
Thailand, Cambodia, Laos and north Vietnam. We have links from Asia/Bangkok to
Asia/Phnom_Penh and Asia/Vientiane for backward-compatibility purposes only, as
the names Asia/Phnom_Penh and Asia/Vientiane would not exist according to
today's guidelines. There's no need for a backward-compatibility link for
Asia/Hanoi since that name never existed in tzdb. These are all technical
reasons, not political ones.