Ask the Rabbi: A matter of time

If I travel from New York to Japan, crossing the International Date Line, on what day of the week should I observe Shabbat?

Halachic questions about crossing the International Date Line (IDL) became pervasive in the 20th century, when fast international journeys became common. With the sun always setting later while traveling westward, a person traversing the globe’s horizon in 24 hours would inevitably return to his departure point at the same time, unless a line arbitrarily marks a new date for that hour.
The necessity of a calendar date line was popularized by 13th century geographers who marveled how a circumnavigator would “paradoxically” gain or lose a date depending on his travel direction. Later, surprised explorers documented how their calendars were off by one day when they landed in distant locations. Various popes and empires subsequently declared calendar date lines in accordance with their needs.
In 1884, a conference established, by convention, the IDL at 180 degrees from the prime meridian in Greenwich, England, dividing Alaska from Russia in the north and New Zealand from Hawaii (and even closer, Tonga from Samoa) in the South Pacific. As such, new calendar dates (e.g. Saturday, January 30) begin at midnight in Tonga, while the previous calendar date (Friday, January 29) simultaneously starts in Samoa, with the result that any calendar date exists at some point on Earth for 48 hours. Subsequently, when a person travels westward across the IDL he “loses a day” by moving forward one date on the calendar (e.g. January 29 to 30), while someone traveling eastward “gains a day” and moves one date backward.
Jewish scholars subsequently discussed whether halachic sources had previously determined a date line and if Halacha should adopt the new convention. The question has major ramifications for Shabbat observance, as halachic sources had long established that one calculates the week, including Shabbat in accordance with the given location under consideration (Shu”t Radbaz 1:76). When is the seventh night of the week in Japan? According to the Friday date established by the IDL, or some other line? Can one leave Hawaii late Thursday evening to arrive several hours later on a day marked in New Zealand as Saturday?
Amazingly, many historians attribute the first historical discussion of a date line to halachic texts. The Mishna declares that the latest hour a judicial court can declare a new month (Rosh Hodesh) is noon (Rosh Hashana 20b). Two 12th-century figures, rabbis Yehuda Halevi (Kuzari II:20) and Zerahia Halevi (Ba’al Hameor), understood that this rule was to ensure that Rosh Hodesh would last 24 hours somewhere on Earth. Since noon is 18 hours after nightfall (which begins the Jewish calendar day), the farthest location which could celebrate a full day of Rosh Hodesh would be 18 hours west of Israel, thereby establishing the date line at 90 degrees east of Jerusalem (today 125° east longitude, which crosses through Australia, China and Russia). Nonetheless, these texts remain somewhat ambiguous and, moreover, many commentators interpreted this mishna in a manner entirely irrelevant to our discussion.
In 1941, when several hundred yeshiva students fled Lithuania, relocating first in Kobe, Japan (135° east longitude, 100° east of Jerusalem) on their ultimate way to Shanghai, they asked different Israeli scholars when to observe Shabbat and Yom Kippur. Rabbi Avraham Karelitz (the Hazon Ish) adopted the Kuzari’s date line, but following indications from lesser-known medieval sources (Yesod Olam), he zigzagged the demarcation line to the easternmost coast of any continuous landmass, thereby keeping all of Australia, Russia and China to the west of the halachic date line, but moving Japan (and New Zealand) to its east. As such, some of the students observed Shabbat on Japan’s Sunday, while donning tefillin on Saturday.
Based on the talmudic source that Jerusalem is the “center of the world,” Rabbi Yehiel Tuketzinsky opined that Jerusalem serves as the prime meridian, with the date line 180° away (145° west longitude). This would keep Japan to the west of the date line, but also move Hawaii to that side of the day, in contrast with the IDL. Lesser-known figures, such as Rabbi Dovid Shapiro, placed the halachic date line in the mid-Pacific, close to the IDL.
Many of the students followed the IDL, based on an earlier ruling givenby Rabbi Moshe Kisilav for World War I refugees, which was laterendorsed by the eminent Rabbi Menahem Kasher. He argued that theearlier sources were at best inconclusive and theoretical, and that theabsence of direct discussion of these issues in previous or subsequenthalachic literature indicates that Jews always followed theconventional date of the broader society. He contended that Jews shouldobserve Shabbat on the day locally reckoned as Saturday, whichrepresents the contemporary practice of Jews living in the region.

The writer, on-line editor of Tradition and its blog Text & Texture (, teaches at Yeshivat Hakotel.

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