The United Nations records Thailand's date of admission as 15-12-1946
Background
The roll of Member States maintained by the United Nations records Thailand's date of admission as 15-12-1946. The same roll carries a date of admission against every Member State's name. ASEAN's own country record for the Kingdom of Thailand gives 8 August 1967 as the commencement date of its membership, the day the Association was established in Bangkok. Thailand was one of the five states that founded ASEAN on that day. Its admission to the United Nations therefore came about two decades before ASEAN existed. Both entries are maintained by the organisations themselves rather than by Thailand.
In plain words
Member States roll United Nations roll of Member States — The list the United Nations publishes of all its members with each one's date of admission.