It was 1999 for Hong Kong.
taken from wikipedia:
in 1983, Hong Kong was reclassified from a British crown colony to a dependent territory. However with the lease of the New Territories due to expire within two decades, the governments of Britain and China were already discussing the issue of Hong Kong's sovereignty. In 1984 the two countries signed the Sino-British Joint Declaration, agreeing to transfer sovereignty to the People's Republic of China in 1997,[25] and stipulating that Hong Kong would be governed as a special administrative region, retaining its laws and a high degree of autonomy for at least fifty years after the transfer. The Hong Kong Basic Law, which would serve as the constitutional document after the transfer, was ratified in 1990, and the transfer of sovereignty occurred at midnight on 1 July 1997, marked by a handover ceremony at the Hong Kong Convention and Exhibition Centre.[25]
I remember the transfer being done with Prince Charles in attendance.
I also remember being in elementarie (broken kee on kee board) school when Jimmie Carter signed the Panama Canal treatie which had this weird date of the last dae of december of 1999 being the dae of the transfer.