[tz] Add new timezone for Hanoi Capital, Vietnam

[Guy Harris]

On Feb 17, 2019, at 6:40 PM, Tim Parenti <tim at timtimeonline.com> wrote:

> As was discussed on the original thread in October 2014, pre-unification time in Hanoi matches Asia/Bangkok since 1970

By which you presumably mean "from 1970 to 1976", because "pre-unification time" ends when North and South Vietnam were reunified in 1976.

> and thus is already considered covered by the database.

If *post*-unification time - which is post-1970 - doesn't match Asia/Bangkok, then Asia/Bangkok should not be used for any part of Vietnam; there should be one or more Asia/{whatever} for Vietnam from 1970 to the present.

If Vietnam's pre-unification offset or rules were different between the north and the south from 1970 to 1976, then "one or more" means "at least two", e.g. Asia/Hanoi and Asia/Ho_Chi_Minh (even if it was called Saigon in 1970).