The Hong Kong stock market is open from 9.30am to 12pm and from 1pm to 4:10pm. The pre-market session starts at 9am. 

The U.S. stock markets are open from 9.30am to 4pm EST. This is 9.30pm to 4am (daylight savings time) and 10.30pm to 5am (standard time). You can place orders for the U.S. market anytime and they will automatically be executed when the market opens.