When flying to New York from London, due to the time zone change you arrive two hours before you left.
When flying from London to New York by Concord, due to the time zones crossed, you can arrive 2 hours before you leave.
More info: A normal subsonic flight takes around 7-8 hours to fly from London to New York, but Concorde's average flight time is around 3 hours 30 minutes, but can be slightly longer if the aircraft needs to hold before landing at the local airports. The fastest ever time for the crossing was 2 hours 52 minutes and 59 seconds.
