In Parshat Bereishit, we find the Torah's definition of a day. ויהי ערב ויהי בקר יום אחד "and there was evening and then there was morning, behold a day." When the definition of a day is viewed from a global perspective, the issue becomes more complex. One of these complexities is the need for an international dateline. This discussion will focus on halachic perspectives on the international dateline.
The phenomenon of the international dateline may be illustrated as follows (as explained by Rabbi David Pahmer). By convention, all countries of the world have agreed to begin each new day at midnight. Since it is not midnight all around the world at the same instant, the new day does not begin simultaneously in all places. When it is midnight in New York, it is only 11 PM in Chicago, and 9 PM in Los Angeles. That means that it becomes Friday in Chicago an hour after it does in New York, and in Los Angeles three hours after N.Y.. Several hours later, it becomes Friday in Hawaii (at Hawaiian midnight) and several hours later Japan follows suit. Eventually, Pakistan, Israel, and France will all begin Friday in turn. After several more hours, it will again be midnight in New York. Does that mean that it will then become Friday?! Surely not, since 42 hours earlier, Friday already set in! Somewhere we must stop saying that it becomes Friday at midnight and say that it now becomes Saturday at midnight. It has been universally accepted to designate as the International Dateline an imaginary line that extends from the North Pole to the South Pole, exactly 180 degrees from Greenwich, England.
Although the halacha recognizes the need for a dateline, it has its own criteria for determining exactly where that dateline is located, which may not coincide with the arbitrary line accepted internationally. The phenomenon of frequent worldwide travel has brought this question to the forefront of halachic discussion, and dozens of articles and entire books have been written exclusively on this important topic written by Rabbi Binyamin Tuemim-Rabinowitz. A review of the literature in English was written by Rabbi David Pahmer (Journal of Halacha and Contemporary Society, Spring 1991, pp. 60-83) and a thorough review of the topic in Hebrew appears in the recently published twenty-third volume of the Encyclopedia Talmudit. Although Rabbi Menachem M. Kasher writes in his book on this topic that there are thirteen opinions on this issue, we will summarize three of these perspectives
1) Rabbi Menachem Mendel Kasher (the editor of the famous work תורת שלמה), in his book, קו התאריך, argues that since there is no explicit Talmudic source for the dateline any arbitrary point is acceptable. Hence, it is permissible to follow the universally accepted international dateline, located 081̊ from Greenwich, England. Rabbi Tzvi Pesach Frank (the late Chief Rabbi of Jerusalem, considered to be one of the outstanding halachic authorities of this century) agrees with Rabbi Kasher's conclusion (Teshuvot Har Tzvi I).
2) Rabbi Yechiel Michel Tukachinsky (one of the outstanding halachic authorities of the previous generation, who resided in Jerusalem) is the main proponent that the dateline should be located 081̊ from Jerusalem. He writes in his book, היומם בכדור הארץ, that the reason for this is that the Torah views ארץ ישראל and Jerusalem as the center of the world (see יחזקאל ל"ח:י"ב and ירושלמי ראש השנה פרק ב'). Therefore, the international dateline from a Jewish perspective should be located 180 degrees from the center of the world - Jerusalem.
3) Rabbi Avraham Yeshaya Karelitz (the Chazon Ish, one of the great scholars of this century who resided in Bnei Brak) ruled that the halacha considers the dateline to be 90 degrees from Jerusalem, Israel, based on the manner in which many Rishonim (Baal Hamaor, Kuzari, Ritva, Ran) understand a passage in Rosh HaShana כ:. They interpret it (as explained by Rabbi David Pahmer) as follows: If the astronomical new moon (molad) occurs slightly before noon (Israel time), then the Beit Din may declare that day as Rosh Chodesh. If the Molad occurs any time after noon, then the next day is Rosh Chodesh. The Gemara further states that this is a consequence of the requirement that we cannot declare Rosh Chodesh unless the newly sanctified Rosh Chodesh will last a full twenty four hours. For example, if the molad occurs in Israel three hours after night fall on Wednesday night (that is, three hours after the beginning of Thursday, since the Jewish day begins at night) the Bet Din can still declare Thursday as Rosh Chodesh, because it is not yet Thursday in New York. Similarly, if the Molad occurs in Israel eight hours after nightfall on Wednesday, then Thursday is still acceptable for Rosh Chodesh, because it is not yet Thursday in California. How long can we continue this pattern? The Gemara in Rosh HaShana כ: designates noon (Israel time) as the latest time for which any locale has yet began the new day. Now, noon is eighteen hours after nightfall, which means that there is still a region in the world where Rosh Chodesh has not yet begun, and that is the farthest west we can travel for which is true.
Accordingly, the Chazon Ish (in an essay entitled שמונה עשרה שעות, which is printed at the conclusion of Hilchot Shabbat in the commentary of the Chazon Ish on Moed) concludes that more than eighteen hours west of Israel is the dateline, and the next day, six hours ahead of Israel.
Some authorities, including Rabbi Yitzchak Zev Soloveitchik (Rabbi Joseph B. Soloveitchik's uncle), accepted the argument of the Chazon Ish and follow it strictly, setting the dateline 90 degrees east of Jerusalem. There is some practical difficulty with this opinion because the 90 degree line runs through Siberia, China, and Australia. The Chazon Ish, however, believes that אין מחלקין היבשות, "we do not divide continents." That means that if any part of a landmass falls within 90 degrees east of Jerusalem, then this entire landmass is considered by the Halacha as if it was within 90 degrees east of Jerusalem. Accordingly, the dateline conforms to the coastline. For example, although part of Australia is west of the 90 degrees line (from Jerusalem), all of Australia would still have the same Halachic status of being in the same day as Israel. Practically speaking, the Chazon Ish's opinion matches the international standard with regard to the major landmasses of Australia, China and Russia. Other areas, as we will soon see, pose a greater problem.
In summary, some argue that Halacha accepts the internationally recognized 180 degrees line from Greenwich, England, others place it at 081̊ from Jerusalem, others place it at 90 degrees from Jerusalem, and the Chazon Ish places it 90 degrees from Jerusalem, but asserts that the dateline conforms to the coastline.
There are great Halachic ramifications to the issue as Japan and New Zealand lie to the west of the secular international dateline but to the east of the 90 degrees line. According to the Chazon Ish, Shabbat in Japan and New Zealand should be observed on what these countries refer to as Sunday. On the other hand, Hawaii lies to the east of the international dateline but to the west of the 180 degrees line from Jerusalem. Accordingly, Rabbi Tukachinsky rules that Shabbat in Hawaii should be observed on what is generally regarded as Friday.
Regarding how one crossing the "dateline" should act, there is a difference of opinion among contemporary Rabbinical figures. Some Poskim counsel avoiding the question altogether by avoiding travel on Fridays and Sundays. Other Rabbinic authorities rule that one may follow the custom of the established Jewish communities of Japan and New Zealand to observe Shabbat on what is regarded as Saturday. It would follow that those who wish to travel to the areas in question should consult their Rabbi for Halachic guidance regarding this complex issue.