Suggestions for show how many days? Passing the International Date Line

Little background Let's say you fly from the United States to Japan and suppose you leave the United States on Tuesday morning. Because you're travelling west, the time advances slowly thanks to time zones and the speed at which your aeroplane flies. But as soon as you cross the international dateline, it's suddenly Wednesday.

Below image from google flight, which is +2 means taking 2days gap. this +2 doesn't make a sense for me.