The international date line[note 1] in Judaism is used to demarcate the change of one calendar day to the next in the Jewish calendar. The Jewish calendar defines days as running from sunset to sunset rather than midnight to midnight. So in the context of Judaism, an international date line demarcates when the line of sundown moving across the Earth's surface stops being the sunset ending and starting one day and starts being the sunset ending and starting the following day.
However, the conventional International Date Line is a relatively recent geographic and political construct whose exact location has moved from time to time depending on the needs of different interested parties. While it is well-understood why the conventional date line is located in the Pacific Ocean, there are not really objective criteria for its exact placement within the Pacific.[1] In that light, it cannot be taken for granted that the conventional International Date Line can (or should) be used as a date line under Jewish law. In practice, within Judaism the halakhic date line is similar to, but not necessarily identical with, the conventional Date Line, and the differences can have consequences under religious law.
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