The International Conference in 1884 deemed that there would be a single Universal Day and that this would begin at mean midnight at Greenwich. Twenty five time zones were established to the east and west of Greenwich with the International Date Line lying along the 180 line of longitude. The line deviates in places to avoid crossing any land. Along this line the calendar moves into a new day but only in local time, which is measured relative to Greenwich Mean Time. Some small islands are moving their territory into a new time zone so that they can claim to be first into the millennium.
Any government is able to define their time and time zones at any time relative to Greenwich.
Countries like China only have a single time zone. We welcome feedback and comments. Please read the legal page and data policy page. Awards and media coverage - credits page.