After Yosef reveals who he really is to his brothers, he instructs them to quickly "go up" (ועלו) to his father Yaakov (who was in Eretz Yisrael) and tell him that he is alive and now a leader in Egypt, and that he should hurry down to see him (בראשית מ"ה:ט'). In his commentary on this Posuk, Rashi (שם בד"ה ועלו), apparently troubled by the word ועלו, go up, used to describe the journey to Eretz Yisrael, writes that Eretz Yisrael is higher than, or above, all other lands; to get there, then, one must indeed "go up." This idea would appear to be based on the Gemara in Kiddushin (דף ס"ט.) which makes this very statement about Eretz Yisrael, adding, based on a Posuk later in the Torah (דברים י"ז:ח'), that the site of the Beis HaMikdash (in Yerushalayim) is higher than any other place in Eretz Yisrael; this latter point, though, is qualified by Tosafos in Sanhedrin (דף פ"ז. בד"ה שבית) where these statements also appear. Although the Pardes Yosef on the Posuk in our Parsha (בראשית שם) quotes those who explain that Chazal's intent here is merely to teach that the Beis HaMikdash and Eretz Yisrael are higher than other places in the world in terms of stature and honor, the Maharsha in Kiddushin (חידושי אגדות שם בד"ה בהמ"ק) writes that the earth is a sphere, and the site of the Beis HaMikdash and Eretz Yisrael are at the center -or on top- of that sphere; the place is thus indeed higher than any other place in the world. He apparently understood the statements of the Gemara (שם) in the literal sense.
This idea that Eretz Yisrael and Yerushalayim are at the "top" of the world, and the idea expressed in the Gemara earlier in Sanhedrin (דף ל"ז. ועיין שם ברש"י ד"ה בטיבורו) that the site of the Beis HaMikdash (in Yerushalayim, where the Sanhedrin had its chambers) is at the center of the world, may impact the issue of how to determine the location of the International Date Line according to the Halacha. It is the accepted practice in the secular world to begin the new calendar day at midnight. Since, however, midnight does not arrive at the exact same moment all over the world, the new calendar day will not begin at the exact same moment all over the world. The further west one travels, the "later" midnight -and the new calendar day- arrives. For example, when Sunday evening concludes and it becomes Monday at midnight in New York, it is still Sunday evening for three more hours in California. And it will not become Monday for another few hours after that in Hawaii, and several hours after that, it would become Monday in the Orient, and so on. Eventually, one will "travel" all the way back to New York, where it will again be midnight. Obviously, though, it cannot again be the midnight which begins Monday, because that already happened 24 hours earlier; this midnight will have to begin Tuesday. It is clear, then, that at some point, somewhere along the line, midnight will no longer be the beginning of Monday, but of Tuesday, and this "cycle" of the new day begins again. This point is what is known as the International Date Line, and at the International Meridian Conference in 1884, it was decided that this date line would be at the point which is 180̊ (that is, twelve hours) from Greenwich, England, a point which cuts primarily through the Pacific Ocean.
The question is, where exactly is this date line according to the Halacha, which obviously assigns no significance to Greenwich, England? Of course, midnight is of no relevance in terms of the date according to the Halacha either, but the same problem naturally presents itself regarding sunset, which obviously is an important time in Halacha. The Radvaz (שו"ת הרדב"ז חלק א' סימן ע"ו) already asserts that this matter is the subject of dispute among Rishonim, and, as documented by Rav Shlomo Zevin (ספר לאור ההלכה, עמוד רפ"ה), many others have dealt at length with this issue. Some point to a Gemara in Rosh HaShanah (דף כ:) which indicates that the Beis Din would not declare a day to be Rosh Chodesh unless the new moon was visible in Eretz Yisrael before noon on that day; if it appeared after noon, the next day would be Rosh Chodesh. Although certain Rishonim explain the Gemara (שם) as referring only to details concerning the declaration of Rosh Chodesh, the interpretation of the Baal HaMaor (שם דף ה. בדפי הרי"ף בד"ה כי) and others relates it directly to this issue of the date line.
The Gemara (שם) states that the reason Beis Din would not declare a day to be Rosh Chodesh unless the moon appeared before noon in Yerushalayim is that this Rosh Chodesh must last a full 24 hours somewhere in the world. In other words, it is required that there be some place on earth that will be experiencing nightfall at the time of (or after) the Molad, the first appearance of the moon in Eretz Yisrael, so that the newly declared Rosh Chodesh will last a full 24 hours in at least that one place. Therefore, the Baal HaMaor (שם) explains, if Rosh Chodesh is declared in Yerushalayim anytime between nightfall and noon (18 hours later), there will be places in the world that will have a full 24 hours of the new Rosh Chodesh. Even if it is noon in Eretz Yisrael at the time of the Molad, in which case only 6 hours remain of Rosh Chodesh there, there is still some place on earth where it is just nightfall, and where Rosh Chodesh will thus last a full 24 hours. If, however, the Molad appears in Eretz Yisrael after noon, there will no longer be any place which will not yet have experienced nightfall of that day; Rosh Chodesh thus cannot be that day because no location would experience it for a full 24 hours. The Gemara here is thus suggesting, according to this interpretation of the Baal HaMaor, that noon in Yerushalayim is the latest time at which there is still some location on earth which has not yet begun the new day. Since noon is 18 hours after nightfall, this means that there is a location far west of Yerushalayim which is 18 hours "behind" it in terms of time. Apparently, more than 18 hours west of Yerushalayim, it is already the next day, 6 hours "ahead" in terms of time. It would thus seem that the Gemara has determined that the date line is 18 hours (270̊) west, or 6 hours (90̊) east of Eretz Yisrael.
Rav Chaim Zimmerman, in a lengthy work on this subject (ספר אגן הסהר, פרק ב' עמוד י"ט), suggests that it may also be because the Baal HaMaor (שם) considers Eretz Yisrael (and Yerushalayim) to be the "top" of the world, as mentioned above, that he holds that the date line must be 90̊ east of Yerushalayim. He explains that at the moment of creation, when the sun was placed in the sky, it was placed above Yerushalayim which, at that first instant, was naturally on "top" of the world in relationship to the sun, as every part of the globe eventually is. This means that the area 90̊ east (6 hours ahead) of Yerushalayim was then experiencing nightfall (sunset) and was already beginning the next day. That location, then, must be the date line because that was the first place on earth to go on to the next day; this indeed is where the Baal HaMaor (שם) places the date line, as shown above.
This position about the date line is articulated as well in the Sefer HaKuzari (מאמר ב' סימן כ'), and is clearly accepted by the Ran (חידושי הר"ן לר"ה שם בד"ה צריך), and, apparently, by the Ritva (חידושי הריטב"א שם בד"ה כי). The Chazon Ish, in an essay on this topic (קונטרס י"ח שעות, נדפס בספר חזון איש לאורח חיים-מועד, סימן ס"ד), asserts forcefully (אות ד',י"ג,ט"ז, ובסיכום שם, ובמכתב שני בסוף הקונטרס) that this is indeed the position of all the Rishonim who discuss this matter. Although Rav Chaim Zimmerman, throughout his Sefer cited above (שם), discusses views which in fact appear to disagree with this position, he suggests (שם פרק י"ב) that the Rambam (בהל' קידוש החודש) may be added to the list of those who subscribe to it. Despite the fact that the Chazon Ish writes elsewhere (קובץ אגרות להחזון איש, חלק ב' אגרת קס"ו) that the issue is simple and that there is no room for debate, there are nevertheless other recent authorities who disagree, as documented by Rav Shlomo Zevin in his aforementioned article (שם עמודים רפ"ז-רפ"ח).
In particular, he cites (שם עמוד רפ"ח) Rav Yechiel Michel Tukatchinsky, author of the Gesher HaChaim, who wrote a Sefer (היומם בכדור הארץ) on this subject (to which the Chazon Ish's קונטרס י"ח שעות is a response, according to its הקדמה). He shows that in fact the new day begins 12 hours to the east of Yerushalayim, and finally ends 12 hours to its west; this means that the date line is 180̊ (in either direction) from Eretz Yisrael. As Rav Zevin notes (שם עמוד רפ"ז), one advantage of this position is that Eretz Yisrael is actually in the center of the world according to this calculation (with 180̊ to the date line on each side of it); this works out nicely, therefore, with the statement of the Gemara in Sanhedrin (שם) cited above that Eretz Yisrael is the center of the world. As reported in the biography of the Chazon Ish (ספר פאר הדור\ חיי החזון איש, חלק ג' פרק כ"ד), at a meeting convened in 1941 by the Chief Rabbinate in Eretz Yisrael to respond to questions of Jews who had fled Europe and were living in the Orient, this position was officially accepted; Rav Tzvi Pesach Frank (שו"ת הר צבי או"ח חלק א' סימן קל"ח) ruled this way as well.
The Chazon Ish, however, continued to insist that his position is correct, writing in another letter (קובץ אגרות שם אגרת קס"ד) that he dislikes getting involved in this type of situation, but feels that he can't keep silent about this issue, especially since, in his view, all the Rishonim support the position that the date line is 90̊ east of Yerushalayim. In another letter (שם אגרת קס"ה), he describes the responsibility he feels to render a decision in consonance with the view of the Rishonim. In terms of Eretz Yisrael being the center of the world, the Chazon Ish (קונטרס י"ח שעות שם אות ב' ,ובמכתב שני שם בסוף) explains that we are to consider only the upper hemisphere of the globe, because that's where Eretz Yisrael is, and where human civilization began and developed. If we place Eretz Yisrael at the "top" -or center- of that originally inhabited hemisphere, the eastern edge is 90̊ east of it and the western edge is 90̊ to its west. The eastern edge is where the new day begins; that point, 90̊ -or 6 hours- east of Eretz Yisrael, is the date line.
One practical problem with this position of the Chazon Ish is that "his" date line, 90̊ east of Yerushalayim, cuts through land masses in eastern Russia, China, and Australia, which means that within one country, or even one city, Shabbos could be on two different days. He explains, however (שם אות ה'), that this would not happen because any land mass which has a significant portion to the west of the date line is considered Halachically to be fully in the area west of the date line, even if it physically is not. In other words, the date line, according to him, "bends", and follows the coastline of those land masses; all of eastern Russia, China, and Australia are thus considered to be east of Eretz Yisrael (ahead in time) and west of the date line. It is noteworthy, though, that according to the Chazon Ish, Japan is undoubtedly to the east of the Halachic date line, and thus is behind Eretz Yisrael in time, even though it is west of the official International Date Line; Shabbos in Japan must thus be observed on their Sunday, not their Saturday. Since there are numerous Halachic questions which can come up when one crosses the date line, such as about Shabbos and Davening, a competent Halachic authority should be consulted.