The International Date Line and the Halachic Date Line, known as Kav Hataarich, are similar in nature, yet they might be in different places on the map. This article will go into the location of the Halachic Date Line and the difference between it and the International Date Line, and some Halachot that apply to one who crosses the Halachic Date Line. For a full Halachic discussion of this issue see the twenty second volume of the Encyclopeida Talmudit.
International Date Line
The International Date Line is an imaginary Line on the 180th meridian in the Pacific Ocean that goes through the Bering Strait (between Alaska and Russia), which is half way around the world from Greenwich, England. It is basically a straight Line; however, there are some zigzags. These zigzags are necessary because otherwise one country would be observing two Dates at the same time. In order to prevent this, the International Date Line curves around these countries and only goes through the Pacific Ocean  (See Fig.1). Its purpose is to be a separation between two consecutive calendar days, the old day and the new day. So if one were to go west of the International Date Line, he would be in the new day. However, if he were to go east of it he would be in the old day. The old day stretches from the International Date Line eastward until it reaches the spot where midnight begins (See Fig.2). The International Date Line, which is “attached” to the Earth, moves towards midnight as the Earth turns. Once the International Date Line reaches midnight, in effect for that moment the entire world is under one day. However, once the International Date Line passes through midnight, the new day begins to spread between midnight and the International Date Line, and the old day gets smaller as the process is being repeated . This is the separation that is accepted in the secular world, but not necessarily in the Halachic World.
Location of the Halachic Date Line
The Halachic Date Line is possibly different than the International Date Line. The reason for this, is that the current International Date Line was rather arbitrarily drawn in 1921 by the British Admiralty , while certain Gedolim such as the Baal Hamaor who lived in the 12th century refer to the Halachic Date Line. So, on the topic as to the location of the Kav Hataarich there is much debate amongst twentieth century Poskim, including the Chazon Ish, Rav Yechiel Michel Tukatzinsky, Rav Isser Zalman Meltzer, Rav Yonassan Shteif, the Sefer Haibur, Rav Dovid Shapiro, and Rav Yonah Merzbach .
The earliest authority to address this issue is the Baal Hamaor. He brings up the topic based on a Gemara (Rosh Hashanah 20b) regarding the topic of Kiddush Hachodesh, the sanctification of the new month. The Baal Hamaor explains that Bait Din has until noon on the day that they see the molad, new moon, to declare Rosh Chodesh on that same day. However, if it is after noon, then Rosh Chodesh is on the next day. This explanation would only make sense if the Halachic Date Line was at the Kitze Hamizrach which is 90° east of Jerusalem. This is so because the reason why the Baal Hamaor said noon is because that is the last time in Israel that somewhere else in the world that the day is just starting. In order for Rosh Chodesh to be on that day, it must be possible for Rosh Chodesh to last 24 hours somewhere in the world. Since noon is 18 hours into the day (starting from sunset on the night before), the place where the day is just starting is 18 hours to the west of Israel which is 270° west of Israel because every time zone is made up of 15°. So, the place where the new day starts, or the Halachic Date Line, must be six hours to the east of Jerusalem which is also 90° east of Jerusalem. This Line is on the 125E meridian .
The modern question, as to the location of the Halachic Date Line, was presented to the Chazon Ish in a letter sent to him in Israel in 1941 by students of Mir and Chachmei Lublin that fled from Europe to Japan. The students knew that according to the Baal Hamaor the Halachic Date Line is on the 125E meridian and Japan is on the 140E meridian. So, they knew that they must have crossed the Date Line, meaning that they went back one day. For example, if they crossed on Sunday then it is now Shabbat in Japan according to Halacha, even though the local population considers the day as Sunday (since it was west of the secular DateLine). The students’ question was regarding Yom Kippur in 1941 which fell out on a Wednesday. If they had indeed crossed the Halachic Date Line, then perhaps they should observe Yom Kippur on the Japanese Thursday which is the Halachic Wednesday. The Chazon Ish responded to the students of the Mir Yeshiva including Rav Yechezkel Levenstein, Rav Chaim Shmulevitz, and Rabbi Alex Weisfogel that they had crossed the Halachic Date Line. According to the Chazon Ish, the principle of the Baal Hamaor is correct; the Halachic Date Line is at the Kitze Hamizrach, but it can not cut through land because then one person will be observing one day while his neighbor is observing another day. So, he says that the Halachic Date Line is at the end of the continent on the border between the coast and the Pacific Ocean so that it does not cut through any land (See Fig.3) . Thus, all of Australia is considered to lie west of the Halachic DateLine even though much of Australia lies east of the Halachic DateLine.
Based on the same question raised by the students of the Mir Yeshiva and Yeshivat Chachmei Lublin, Rav Yechiel Michel Tukatzinsky, who was the leading calendar expert in Israel at the time, answered that the students of the Yeshivat Chachmei Lublin should fast on the Japanese Wednesday because they did not cross the Halachic Date Line. Rav Tukatzinsky rules that the Halachic Date Line is based on Jerusalem being the center of the world and it is the starting point where east and west is referred to. So, the spot on the Earth where the day Halachically starts is half away around the world, 12 hours or 180° east of Jerusalem. This is the Halachic Date Line which is at the 144.8W meridian (See Fig.3). This is the most widely accepted opinion as to the location of the Halachic Date Line, but according to him, half of Alaska is west of the Date Line. So, by using the Chazon Ish’s principle of stretching the Line on to the border, all of Alaska is east of the Halachic Date Line . In addition, Hawaii lies west of the Halachic Date Line according to this opinion, even though it is west of the secular Date Line.
Rav Isser Zalman Meltzer and Rav Zvi Pesach Frank (Teshuvot Har Zvi 1:138) claim that there is no such thing as a Halachic Date Line. Instead, a person just follows the day that the country that he is in is observing. For example, if in Japan it’s Wednesday, then the person views the day as the halachic Wednesday . So, they would answer the students that they should observe Yom Kippur on the Japanese Wednesday. Also, since the world adopted the International Date Line as the point where the day begins, in essence they would say that this too is the Halachic Date Line. Rav Yonatan Shteif would also probably agree based on the fact that he says that the Halachic Date Line is based on the day that the country traditionally observes .
The Sefer Haibur says that the center of the world is 24° east of Jerusalem. So using the principle of the Baal Hamaor, that the new day starts six hours east of the starting point, the Halachic Date Line is 114° east of Jerusalem, which is on the 149E meridian (See Fig.3) .
Rav David Shapiro believes that the Halachic Date Line is very close to the International Date Line. There is a Midrash that says that the sun first appeared in Jerusalem in the beginning of the fourth hour. So, the sun first appeared in the world three hours or 45° east of Jerusalem. However, the day starts at sunset, which is another six hours or 90° east. In total, sunset, or the beginning of the first day, took place nine hours or 135° east of Jerusalem. However, most poskim hold that a day halachicly begins at nightfall, tzeit hakochavim, which is approximately 8° east of sunset. In total, the spot where day began on the first day is 143° east of Jerusalem, which is on the 178E meridian. This is within two degrees of the International Date Line, which is on the 180th meridian. So basically according to Rav David Shapiro, the Halachic Date Line is the International Date Line .
According to Rav Yonah Merzbach and Rav Binyamin Rabinowitz- Tevmim, the Halachic Date Line is at the easternmost point of Asia, which is the tip of Siberia and the Bering Strait. This Line is on the 170W meridian, which is ten degrees east of the International Date Line (See Fig.3) .
Although there are many opinions as to the location of the Halachic Date Line, most Poskim consider three possibilities when applying halachic decisions. Either the Chazon Ish, which is the Baal Hamaor but the extension to the coast is added, Rav Tukatzinsky, and Rav Isser Zalman Meltzer.
There are various areas of Halacha emerge from the Halachic Date Line issue. There are many mitzvot that are related to time and day that are effected by someone who crosses the Halachic Date Line. Three of the most common ones are Tefila, Sefirat Haomer, and Tefilin. However, in order to discuss these topics, one specific Halachic Date Line must be used. Even though there are various opinions as to the location of the Halachic Date Line, Rav Tukatzinsky’s is the most widely accepted one. So, that is the one that will be taken into consideration when discussing the crossing of the Halachic Date Line. However, each one of these mitzvot can be discussed for each opinion.
Because Tefila or prayer occurs at least three times every day, it is one of the most frequently brought up questions when dealing with Date Lines. There are two related questions that are based on the direction in which the Date Line was crossed. First of all, a person traveling from east to west would be going into a new day. For example, if he left Los Angeles on Sunday at 6:00 AM, when he gets to Tokyo it will be 11:00 AM on Monday because of the 17 hour time difference and the 12 hour flight . So, if he davened Shacharit on Sunday in Los Angeles, must he daven Shacharit on Monday in Tokyo even though he has not experienced a new sunrise? Another question that arises is from a similar scenario. A person traveling from west to east would be going back one day, and might experience sunrise as after he crosses the Date Line. For example, if a person leaves Tokyo at 3:00 PM on Monday he will get to Los Angeles at 7:00 AM on Monday morning because of the 17 hour time difference and the nine hour flight . So, during his flight he experienced sunrise on Monday morning for a second time, yet he already davened Shacharit on Monday morning. Must he daven Shacharit again? 
Regarding the first case Rav Betzalel Stern in his sefer, Teshuvot Betzel Hachochma, explains that one’s obligation to daven is based on their personal day. In other words, every time a person experiences sunrise they are obligated to daven Shacharit, and every time they experience sunset they are obligated to daven Ma’ariv. In this case, since he did not experience a new sunrise, he does not have to daven Shacharit a second time. A second opinion is given by Rav Yechezkel Roth in sefer, Teshuvot Emek Hateshuva, who says that one’s obligation to daven Shacharit is only once a day, and once it has been fulfilled the next obligation only comes the next day. For example, the obligation to eat Matzo on Pesach is only once a year, and once it has been completed the next obligation only comes the next year. So, in this case even though he did not experience sunrise, his obligation to daven Shacharit on Monday was not fulfilled, and he must daven Shacharit again .
On the other hand, regarding the second case Rav Stern says that since he did experience sunrise he must daven Shacharit again in order to fulfill his obligation. However, Rav Roth says that since it is still the same day of the week (Monday), even though he experiences two sunrises, he does not have to daven Shacharit again . For practical Halacha, anytime one experiences a sunset and then a sunrise must daven Shacharit after sunrise. However, if one does not experience a sunrise then he should just recite the Shir Shel Yom for the day that he flies into .
There is a mitzva in the Torah to count 49 days at night with a bracha, beginning with the second night of Pesach. If one forgets to count at night, he may count in the morning without a bracha and then can continue to count from that night on with a bracha. However, the Shulchan Aruch rules that if one goes an entire day and night without counting, he can no longer count with a bracha. So, if one were to cross the Date Line from east to west, he will be going into a new day and one day is lost. For example, a person who leaves New York on Sunday night and counts for Monday on Sunday night (as is the Halacha), and then will reach Tokyo on Tuesday morning (as there is a 14 hour time difference and it is a 18 hour flight), should count for Tuesday on Tuesday morning when he crosses the Date Line without a bracha, and then continues to count that night with a bracha. However, if one crosses the Date Line from west to east, the day repeats itself. For example, one who leaves Tokyo on Monday afternoon will get to Los Angeles on Monday morning. Since he counted on Sunday night for Monday in Tokyo, when he gets to Los Angeles he should count for Monday without a bracha and then continues counting on Monday night with a bracha .
There is a mitzva in the Torah to wear Tefilin, and they are worn every day. However, questions arise when one crosses the Date Line and is not sure whether or not he should put on Tefilin again. In order to answer the question there are two ways to look at the mitzva of Tefilin. If it is a daily mitzva that is only done once a day, then it is subject to the disagreement between Rav Stern and Rav Roth regarding tefila. Tefilin is simply substituted for tefila, and the Halacha would be that any time sunrise is seen Tefilin are worn. Regarding a bracha, a bracha is said every time Tefilin are put on because they can be worn many times during the day. So, one who is crossing the Date Line from east to west, from Los Angeles to Tokyo, should put on Tefilin when he crosses the Date Line of it is light outside. The reason for this is because if there is doubt as to whether or not one should put on Tefilin, it is okay to put them on because they can be worn many time during the day. If one is crossing the Date Line from west to east, from Tokyo to Los Angeles, he should only put on Tefilin, if he experiences sunrise because other wise he already put them on once after a sunrise, i.e. before he left Tokyo .
The International Date Line and the Halachic Date Line might be in two different places, but they both serve a similar purpose. They both separate between two different days. In the world of Halacha it is very important to know where the Halachic Date Line is because so many mitzvot are attached and dependent on time. Tefila, Sefiras Haomer, and Tefilin are just three of the most common ones that are part of a long list. One should consult with his Rav should he need to travel to any part of the world that is subject to this dispute.
 U.S. Naval Observatory, The International Date Line, http://aa.usno.navy.mil/faq/docs/international_Date.html, 12/3/02, 12:43 PM.
 Goals, Inc., Sailing thru Science… International Date Line, http://www.goals.com/sailscin/DateLine.htm, 12/3/02, 12:47 PM.
 Zalman Tropper and Rav Yisroel Taplin, The Date Line in Halacha, Lakewood, NJ, 1999.
 David Pahmer, The International Date Line and Related Issues, The Journal of Halacha and Contemporary Society Number XXI, Staten Island, NY, 1990.
 The time lengths of the flights were given by Continental Air Lines.
 The psak was given by Rabbi Steven Pruzansky of Congregation Bnai Yeshurin in Teaneck, NJ.