Five and a half years. The Hang Seng Index is an important indicator of the price of the Hong Kong stock market. Based on the market value of several constituent stocks, the longest futures contract is five and a half years. The Hang Seng Index is compiled by Hang Seng Index Service Co., Ltd., a wholly-owned subsidiary of Hang Seng Bank in Hong Kong, and takes 50 listed stocks in the Hong Kong stock market as a sample of constituent stocks.