Prior to 1 January 1901 locations in Siam with an astronomical observatory would adopt local mean time based on the observatory's geographic position. Chiang Mai Province and two other provinces each had an observatory, hence, each province had its own distinct local mean time, with minutes of difference between the three locations. On 1 April 1920 the mean time of the 105th meridian east (passing through Ubon Ratchathani Province) was adopted by Siam as the new standard time.