Vietnam timezone history corrections

[Alois Treindl]

It seems necessary to split North Vietnam and South Vietnam into two 
zones for timezone history between 1967 and 1975/1976

I have found corroboration that North Vietnam went from UTC+8 to UTC+7 
on August 8, 1967, while South Vietnam stayed on UTC+8 'until after the 
end of the Vietnam war'. Saigon fell end of April 1975.

Source: http://en.wikipedia.org/wiki/T%E1%BA%BFt

There is another reference, discussing the difference between North and 
South Vietnam timezone in 1968,
http://www.math.nus.edu.sg/aslaksen/calendar/cal.pdf
on page 29.
The timezone difference led to a different date for the Tet festival in 
that year, creating confusion about the starting data for the Vietcong's 
Tet offensive.