Pick your country or city, or search by UTC offset. Every time on the site follows it: the economic calendar, charts, the Market Pulse timeline, the trading session timeline, the countdown to the US open and older news. You can also change it from the clock button in the top bar. Whether the market is open is always decided by exchange time (America/New_York), whatever you choose here.
Sign in and alert notifications will use it too; without an account they quote New York time.