Thailand follows UTC+07:00, which is 7 hours ahead of UTC. The local mean time in Bangkok was originally UTC+06:42:04. Thailand used this local mean time until 1920, when it changed to Indochina Time, UTC+07:00; ICT is used all year round as Thailand does not observe daylight saving time. Thailand shares the same time zone with Vietnam, Cambodia, Laos, Christmas Island, and Western Indonesia.
|Period in use||Time offset from GMT||Name of time (unofficial)|
|1 January 1880 - 31 March 1920||UTC+06:42:04||Bangkok Mean Time|
|1 April 1920 – present||UTC+07:00||Indochina Time (ICT)|
Thailand declared on 16 March 1920 that people would move their clocks ahead by 17 minutes, 56 seconds on 31 March 1920 to match the time in use in Southeast Asia. The time was switched on 1 April 1920 at 00:00 (old time) to 00:17:56 (new time).
On 1 January 1990, the Cabinet of Thailand appointed the Royal Thai Navy as the official timekeeper for Thailand. Thai Standard Time is derived from five atomic clocks maintained by the Royal Thai Navy.