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 (or requests) of varied countries in the Pacific Ocean. Note how it bends to include all of Kiribati, Samoa, Tonga and Tokelau in the Eastern Hemisphere.
Immediately to the left of the International Date Line the date is always one day ahead of the date (or day) immediately to the right of the International Date Line in the Western Hemisphere.
On the time and date codes shown below, note that Tonga and American Samoa have the same time but are one day apart, as American Samoa is in the Western Hemisphere, on the opposite side of the International Dateline from Tonga. A global Earth view is here.
As you travel further west, note that the time in Fiji is one hour earlier than Tonga. You will also notice that Hawaii, further to the east of American Samoa, is one hour later in time.
So, travel east across the International Date Line results in a day, or 24 hours, being subtracted. Travel west across the International Date Line results in a day being added.
UTC and GMT an update
The worldwide standard for coordinated universal time, formerly known as GMT (Greenwich Mean Time), is now abbreviated as UTC (Coordinated Universal Time).
Some sources still refer to it as UTC/GMT and some web browsers, e.g. Mozilla Firefox, still display it as GMT.
The UTC Time (shown directly below) is the standard time zone upon which all other worldwide time zones are based. It never observes daylight saving time.Additional details on world timezones
UTC at Greenwich, England
FJT + 12 hours from UTCFJST + 13 hours from UTCFiji
TOT + 13 hours from UTCTonga
SST - 11 hours from UTCWST - 10 hours from UTCAmerican Samoa
HAST - 10 hours from UTCHawaii
- FJT Fuji Time
- HAST Hawaii Standard Time
- TOT Tonga Time
- SST Samoa Time
- DST Daylight Savings Time