As we fly west from Israel, we keep moving the hour back, so as not to outpace the sun. But if we were to constantly do that, we could end up arriving back in Israel at an earlier time and date than having left it! Hence, the need for an international dateline. But where is the halachic international dateline?
This is a famous and complex question that I certainly can't do justice to in a short article. But I will outline the issues and mention the factors that are relevant from a rationalist Jewish viewpoint.
There are three well-known approaches to this question, the first of which has two variants. The first approach is based on an inference from the words of the Baal HaMaor, which are in turn an inference from the Gemara in Rosh HaShanah relating to when the new moon can be seen. This is a very technical discussion, but the bottom line is that according to this view, the day begins six hours (90°) east of Jerusalem. According to the strict interpretation of this, followed by the Brisker Rav, China and parts of Russia and Australia would be west of the halachic dateline, i.e. people in those places would observe Shabbos on what the rest of the world considers to be Sunday.
A variant on this approach is that of the Chazon Ish. He considers it unreasonable for the halachic dateline to bisect a country – it would mean that your next-door neighbor could be keeping Shabbos on a different day than you! Hence, he says that contiguous land of China, Russia and Australia should be incorporated to their western parts. According to this, only places such as New Zealand would be keeping Shabbos on what the rest of the world considers Sunday.
Some, however, would entirely reject these approaches. This is because they consider the inference from the Baal HaMaor's inference to be either technically incorrect, or unsuitable for resolving a question that Chazal were not addressing.
A second approach is that of Rav Yechiel Michel Tucazinsky. He bases himself of the Gemara's statement that Jerusalem is the navel, i.e. center, of the world. This is understood to mean that it is the Prime Meridian. Accordingly, the halachic dateline is 180° east/west of Jerusalem. This is extremely close to the secular international dateline – the significant difference being the Hawaiian islands. These would on the western side of the halachic dateline rather than the eastern side, and thus eleven hours ahead of Israel rather than thirteen hours behind; accordingly, Shabbos would be on Friday.
But while it may seem intuitive to use this Gemara to resolve the question, it is problematic. The meaning of the Gemara's statement that Jerusalem is the "navel" of the world is not at all clear. Even if it is making a geographic rather than spiritual statement, it was stated at a time when the conception of world geography was very different. In fact, when the Americas were discovered, R. David Gans felt that this Gemara posed a problem, and felt forced to explain it as a geo-cultural statement that Jerusalem is the center of the civilized world (or something like that; it's a long time since I saw it).
A third approach is that of Rav Isser Zalman Meltzer and Rav Zvi Pesach Frank. They are of the view that there is no halachic Prime Meridian and thus no unique halachic international dateline. Rather, one simply follows what the rest of the world considers to be the day of the week. It would seem that this would be the correct approach from the rationalist perspective.
However, there is one further wrinkle. What about being choshesh lechol hedeyos – being concerned for all opinions? After all, we are talking about Shabbos – a very serious matter! Perhaps Jews in eastern Australia should avoid melachah on Sunday, and Jews in Hawaii should avoid melachah on Friday?
The answer to this also relates to rationalist vs. mystical approaches to Judaism. According to the mystical approach, there is a metaphysical reality to Shabbos, an objective spiritual state that is "out there". Hence, one would probably want to make absolutely sure to be in line with it, and one would take into account other views; after all, they might be right. According to the rationalist approach, on the other hand, there is no independent metaphysical reality to Shabbos. Rather, Shabbos attains its status as a result of how we conduct ourselves.
Of course, there is much more to be said on topic about the international dateline and halachah. It would be great if someone would write a full halachic treatment of this topic from a rationalist standpoint. I can't do this myself, but I would like to point out some things that such a work should take into account:
1) Chazal (at least, those in Babylonia) were of the view that the world is basically flat, with a slight rise to Israel and Jerusalem at the center. (See my monograph The Sun's Path At Night for sources.)
2) The Rishonim, for the most part, knew that the world is a sphere. However, they believed that the lower half was entirely uninhabited. To quote a comment by R. David Ohsie:
The Rishonim, like others, made the assumption that the inhabited part of the world spanned approximately 12 time zones. Naturally, the eastern edge had the earliest times and the western edge had the latest times. There was no need for a "dateline" per se because civilization did not wrap around the globe. The question of where exactly the day turns would be completely theoretical and probably was not considered important; it certainly had no meaning in halacha.
3) Most recent halachic authorities to weigh in on the topic of the dateline probably did not realize/ accept the previous two points. (A notable exception would be R. Menachem Kasher.)
4) To what extent can a halachic dateline be implemented? Here is another fascinating comment from R. David Ohsie:
I want to point out one other huge problem with any "degree" based dateline, especially ones that are close to Asia. We have a general principle that the Torah can be applied with the technology available in ancient times. Anything that requires modern technology, such as a microscope, is not considered imperative. Now in ancient times, there was no way to measure latitude accurately, nor generally to map the extent of landmasses. So to say that the halacha requires you to know that Indonesia is less than 90* from Jerusalem while Japan is greater than 90* or that the western tip of Australia is less than 90* from Jerusalem while you are sitting in the eastern side would be beyond what the halacha required. So more than the fact that the Torah never says where the dateline is, it could not have required a dateline, since that would require the knowledge of modern technology to implement.
5) There is a basic distinction between the mystical and rationalist schools of thought regarding concepts such as sanctity, whether of items, rituals or dates. According to the mystical school of thought, the sanctity of these things exists as an actual metaphysical entity. According to the rationalist school of thought, on the other hand, the sanctity of these items is a state of designation. For extensive and excellent discussion, see Menachem Kellner, Maimonides' Confrontation With Mysticism (which is the most fundamentally important book for anyone interested in rationalist Judaism).
This article was adapted from “No, I Am Not Desecrating Shabbos” and “Rationalism and the International Dateline,” posts which originally appeared on Rabbi Natan Slifkin’s website, www.RationalistJudaism.com