Greenwich Mean Time (GMT) is zero degrees longitude and the world’s time zones are measured east or west of this line. The term Coordinated Universal Time (UTC) is now widely used instead.
The International Date Line crossing the Pacific Ocean changes the date by one day. If you cross the date line moving east, you subtract a day; if travelling west you add on a day (with local variations). Travelling direct from Los Angeles or London to Australia you arrive two days later; travelling back, you arrive the same day you departed. That is, if you ever want to leave.