It is also known as the line of demarcation. It is not straight but zigzags to avoid political and country borders and to not cut some countries in half. When you cross the International Date Line from west to east, you subtract a day, and if you cross the line from east to west, you add a day. Depending on which time zone the country follows, the time difference on either side of the line is not always 24 hours.
For example, if you travel the kilometers miles across the dateline from Baker Island to Tokelau you have to add 25 hours, or 1 day and 1 hour. Changing Sides of the Dateline The dateline is not defined by international law.
Countries are free to choose the date and time zone that they want to observe.
For example, when the Republic of Kiribati gained independence from being a British colony in some of the islands were on one side of the dateline, and the rest were on the other.
They corrected the anomaly in the eastern half of Kiribati by skipping January 1, and ever since Kiribati has been the first country to enter the New Year.
They did this to facilitate trade with Australia and New Zealand, and Tokelau followed Samoa for the same reasons.