International date line in Judaism

The international date line 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 sundown to sundown 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 sundown ending and starting one day and starts being the sundown 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. There are no objective criteria for its placement. 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.

Many of the opinions about the halakhic date line are structured as a response to the question of what days someone should observe as Shabbat and Jewish holidays. Shabbat occurs every seven days at any location on earth. It is normally thought to occur on Saturday—or more precisely, from Friday at sundown to Saturday at nightfall. But if the halakhic date line is not identical to the conventional Date Line, it is possible that what is Saturday with respect to the conventional Date Line is not Saturday with respect to the halakhic date line, at least in some places.

There are several opinions regarding where exactly the halakhic date line should be according to Jewish law, and at least one opinion that says that no halakhic date line really exists.

1. 90 degrees east of Jerusalem. The concept of a halakhic date line is mentioned in the Baal HaMeor, a 12th-century Talmudic commentary, which seems to indicate that the day changes in an area where the time is six hours ahead of Jerusalem (90 degrees east of Jerusalem, about 125.2°E, a line now known to run through Australia, the Philippines, China and Russia). This line, which he refers to as the K'tzai Hamizrach (the easternmost line), is used to calculate the day of Rosh Hashanah, the Jewish New Year. According to some sources it is alluded to in both the Babylonian Talmud (Rosh Hashanah and Eruvin) and in the Jerusalem Talmud. The Kuzari of Yehuda Halevi, also a 12th-century work, seems to agree with this ruling.

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