After Phủ Liễn Observatory was built, French Indochina announced all states (consisting of north-Vietnamese Tonkin, central-Vietnamese Annam, south-Vietnamese Cochinchina, as well as Cambodia, Laos and Chinese Guangzhouwan) were part of 104°17′17″E longitude east of Paris meridian 2°20′14″E, or 106°37′30″E from Greenwich Mean Time from 00:00, 1 July 1906 onward.
In 1911, Metropolitan France adopted UTC±00:00 (the solar time of Greenwich) as its official time, and used it until 1940 (with UTC+01:00 used during the summers from 1916 to 1940), forcing French Indochina observed UTC+07:00 from 00:00, 1 May 1911.
Following change in time zone of Vichy authorities, French Indochina was integrated to UTC+08:00 skipping 60 minutes at 23:00, 31 December 1942.
Under control of State of Vietnam, South Vietnam adopted UTC+07:00 from 00:00, 1 July 1955.
South Vietnam time zone was changed to UTC+08:00 from 23:00, 31 December 1959, passing 60 minutes.
North Vietnam confirmed official UTC+07:00 from 1 January 1968.
Following the Fall of Saigon in April–May 1975, reunified Vietnam then observes UTC+07:00 with Saigon (and other southern parts) delaying 60 minutes on 13 June 1975.