During Northern Hemisphere summer, the EDT time zone in New York is UTC-4, and the AEST time zone in Sydney is UTC+10. That’s a 14-hour time difference between New York and Sydney. When New York is “closing” at 5 pm New York time on Monday (for example), it is 7 am Sydney time on Tuesday. So, if you consider Trading Sessions to be 8 am to 5 pm local time everywhere in the world, then
New York “closes” before Sydney “opens”, during New York summertime.
I put quotation marks around “closes” and “opens”, because technically markets are open around the clock, 24 hours per day, everywhere in the world. What we call a Trading Session is simply the portion of each 24-hour day when trading volume is highest.
During Northern Hemisphere winter (which is Southern Hemisphere summer), the EST time zone in New York is UTC-5, and the AEDT time zone in Sydney is UTC+11. That’s a 16-hour time difference between New York and Sydney. During this time of year, when New York is “closing” at 5 pm New York time (say, on Monday), it is 9 am Tuesday in Sydney, and the forex market there has been “open” for an hour.
So, nominally, during Northern Hemisphere winter, there is a one-hour overlap between the last hour of the New York session and the first hour of the Sydney session.
But, don’t place too much importance on this overlap. The last hour of the afternoon in New York is not marked by high trading volume, compared to, say, the London/New York Overlap period. As the trading day in Sydney is ramping up, traders there are less focused on what’s winding down in New York, and more focused on what the rest of Asia is likely to do, as markets in Tokyo, Singapore and Hong Kong prepare to open (one or two hours after the Sydney market has opened). Bear in mind that the Tokyo, Singapore and Hong Kong markets – which are almost totally overlapped – dwarf the Sydney market in trading volume.