The world is divided into 24 time zones. There is a place where a day begins: The International Date Line:
The International Date Line sits on the 180º line of longitude in the middle of the Pacific Ocean, and is the imaginary line that separates two consecutive calendar days.
It is not a perfectly straight line and has been moved slightly over the years to accommodate needs of varied countries in the Pacific Ocean. Note how it bends to include all of Kiribati in the Eastern Hemisphere.
In the Eastern Hemisphere, left of the International Date Line (the date) is always one day ahead of the date (or day) in the Western Hemisphere.
On the time and date codes shown below, note that Tonga and Samoa have the same time but are (1) day apart, as Samoa is in the Western Hemisphere, on the opposite side of the International Dateline from Tonga.
As you travel further west, note that the time in Fiji is (1) hour earlier than Tonga. You will also notice that Hawaii, further to the east of Samoa, is (1) hour later in time.
Travel west across it (gain a day), travel east across it (lose a day).
Reference: WorldAtlas.com: International Date Line.