What is the law if one crossed the international dateline and went back or forward a weekday?[1]

• Example 1-Traveled from California To Australia: One traveled from California to Australia on Sunday night, the 15^th day of Omer, on a 11:00 PM flight. After 16 hours of flight travel, he arrived in Australia at 3:00 PM Monday California time. However, in Australia, the current time is 9:00 AM Tuesday, and is the 16^th day of Omer! [The time difference between California and Australia is approximately 17 hours, with Australia being 17 hours ahead.] When Maariv of Tuesday night arrives, what day of Omer is the traveler to count, the 17^th, like Australia, or the 16^th like California?
• Example 2-Traveled from Australia to California: One traveled from Australia to California on Monday night, the 15 day of Omer, on a 8:00 PM flight. After 16 hours of flight travel he arrived in California at 12:00 PM Tuesday Australia time. However, in California, the current time is 7:00 PM Monday, and is the 15^th day of Omer! [The time difference between California and Australia is about 17 hours, with Australia being 17 hours ahead.] For Maariv of Monday night, what day of Omer is the traveler to count, is he to repeat the 15^th like California, or is he to count the 16^th like Australia?

Example 1: Some Poskim[2] rule one is to count the 16^th day without a blessing and then count the 17^th with a blessing, just like Australia. On every subsequent night, he only counts like Australia. Others[3] rule he is to count the 17^th, like Australia, without a blessing. So too, on each subsequent night, he is to count without a blessing, being he missed the 16^th day of count. The Rebbe[4] rules that one is to hear the blessing from another person[5] and then count the 16^th, which is his main count, and then sometime later count the 17^th without a blessing. The same applies for each subsequent night, with his California count being his main number of count. In such a case, this traveler is to celebrate Shavuos on the 50^th day of his main count, and hence he begins to celebrate Shavuos one day later than the inhabitants of Australia.[6] He is to wear Tefillin on the 6^th of Sivan just like a weekday, and is to keep Shavuos on the 7^th-8^th of Sivan.[7] He does not recite the words “Zeman Matan Toraseinu” in his Shavuos Shemoneh Esrei.[8] 

Example 2: Some Poskim[9] rule he is to recount the 15^th day without a blessing and then on every subsequent night he counts with a blessing just like California. The Rebbe[10] rules that one is to hear the blessing from another person and then count the 16^th, which is his main count, and then sometime later count the 15^th without a blessing. The same applies for each subsequent night, with his Australia count being his main number of count. In such a case, this traveler is to celebrate Shavuos on the 50^th day of his main count, and hence he begins to celebrate Shavuos one day earlier than the inhabitants of California.[11] He is to keep Shavuos on the 5^th-6^th of Sivan and put on Tefillin on the 7^th of Sivan like a regular weekday.[12] He does not recite the words “Zeman Matan Toraseinu” in his Shavuos Shemoneh Esrei of the 5^th of Sivan.[13] 

Does this law apply likewise to a woman, or to a person who was not counting Sefira?[14] Yes. Even if one did not count Sefira, he is to follow the above-mentioned law regarding his date of celebration of Chag Hashavuos.[15]

Avoiding travel:[16] According to all opinions, one is to initially avid traveling during the period of Sefira in a flight pattern that will add or subtract from him a day of Sefira. This applies for both men and women.

Did not cross dateline but diminished or added hours:[17] One who traveled to a western or eastern time zone, and hence shortened or lengthened his day, is to count Sefira with a blessing when nighttime arrives in his destination, irrelevant of whether he had more or less than 24 hours in his previous day’s count.

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[1] See Likkutei Sichos 3:997; Igros Kodesh 3:119; 17:153, 265, 358; Sichas and letters of the Rebbe printed in Shulchan Menachem 3:18-28; Piskeiy Teshuvos 489:6

[2] Mishneh Halachos 10:121

[3] Betzel Hachahmah 5:96-97; Beir Moshe 7 Elektri 90

[4] Likkutei Sichos 3:997 views his own count as main:only count; Igros Kodesh 3:119 [views both counts as true Safek]; 17:153  and 265 [Says to count both days] [printed in Shulchan Menachem ibid]

[5] Igros Kodesh 3:119 [must count without a blessing]; 17:153 [Tzaruch Iyun if may say blessing] and 265 [Best to hear blessing from another], printed in Shulchan Menachem ibid

The reason: The Rebbe ibid writes that one is to hear the blessing from another being that there is room to doubt which day of Sefira it is for him and it requires further analysis if one can say a blessing on a questionable count.

[6] Likkutei Sichos 3:997; Igros Kodesh 17:153 and 265 [brought in Shulchan Menachem ibid]; See Kinyan Torah 5:46

The Rebbe’s opinions: In the first letter that the Rebbe wrote on this subject in Igros Kodesh 3:119, the Rebbe is left in doubt as to which day of Sefira he must count and hence suggests that one is to celebrate Shavuos due to doubt on the same days as his location, although is to put on Tefillin on the first day of Yom Tov, count the 49^th day of Omer, and not get an Aliya on either day. However, in a later talk, published in Likkutei Sichos ibid, the Rebbe concluded that his main count is his personal count and hence he must celebrate Shavuos on his personal date, which is a day later than Australia. This ruling was later emphasized in letters in Igros Kodesh 17:153 and 265 although the Rebbe concluded there by sending the asker to receive this ruling from at least two other Rabbanim. 

The reason: As the Torah set Shavuos for the 50^th day of Sefira, and not on the 6^th of Sivan [Admur 494:1; Chok Yaakov 494:1] and hence if one’s 50^th day falls before or after the 6^th of Sivan, that is when he is to celebrate Shavuos, irrelevant of the day of celebration of his current location. Tzaruch Iyun from Admur 600:1 in parentheses who writes that in times of Kiddush Hachodesh they would send emissaries to notify the public of the day of Rosh Chodesh Sivan. This implies that Shavuos is set to the 6^th day of Sivan and not the 50^th day of count, as otherwise, what necessity is there for them to know which day was the 1^st of Sivan, either way they will keep Shavuos on the 50^th day from the 16^th of Nissan. Vetzaruch Iyun!

[7] See Likkutei Sichos 3:997

[8] Likkutei Sichos 3:997

Other opinions: The Divrei Nechmia Hashlamos 581 Kuntrus Achron suggests that the reason we say Zeman Matan Toraseinu in Davening, and connect Shavuos with Matan Torah is because in truth the Torah was meant to be given on the 50^th day of the Omer, which is the Shaar Hanun of Bina, and it is due to alternative factors that in the first year the giving of the Torah was delayed until the next day. Accordingly, each year, on the 50^th day of the Omer, the revelations of Matan Torah take effect irrelevant to the day of the month, and therefore even if Shavuos falls on the 5^th or 7^th of Sivan one is to say Zeman Matan Toraseinu. The Rebbe ibid negates his opinion.

[9] Betzel Hachahmah 5:96-97; Beir Moshe 7 Elektri 90

[10] Likkutei Sichos 3:997 views his own count as main:only count; Igros Kodesh 3:119 [views both counts as true Safek]; 17:153 and 265 [Says to count both days] [printed in Shulchan Menachem ibid]

[11] Likkutei Sichos 3:997; Igros Kodesh 17:153 and 265 [brought in Shulchan Menachem ibid]; See Kinyan Torah 5:46

The Rebbe’s opinions: In the first letter that the Rebbe wrote on this subject in Igros Kodesh 3:119, the Rebbe is left in doubt as to which day of Sefira he must count and hence suggests that one is to celebrate Shavuos due to doubt on the same days as his location, although is to put on Tefillin on the first day of Yom Tov, count the 49^th day of Omer, and not get an Aliya on either day. However, in a later talk, published in Likkutei Sichos ibid, the Rebbe concluded that his main count is his personal count and hence he must celebrate Shavuos on his personal date, which is a day later than Australia. This ruling was later emphasized in letters in Igros Kodesh 17:153 and 265 although the Rebbe concluded there by sending the asker to receive this ruling from at least two other Rabbanim. 

The reason: As the Torah set Shavuos for the 50^th day of Sefira, and not on the 6^th of Sivan [Admur 494:1; Chok Yaakov 494:1] and hence if one’s 50^th day falls before or after the 6^th of Sivan, that is when he is to celebrate Shavuos, irrelevant of the day of celebration of his current location. Tzaruch Iyun from Admur 600:1 in parentheses who writes that in times of Kiddush Hachodesh they would send emissaries to notify the public of the day of Rosh Chodesh Sivan. This implies that Shavuos is set to the 6^th day of Sivan and not the 50^th day of count, as otherwise, what necessity is there for them to know which day was the 1^st of Sivan, either way they will keep Shavuos on the 50^th day from the 16^th of Nissan. Vetzaruch Iyun!

[12] See Likkutei Sichos 3:997

[13] Likkutei Sichos 3:997

Other opinions: The Divrei Nechmia Hashlamos 581 Kuntrus Achron suggests that the reason we say Zeman Matan Toraseinu in Davening, and connect Shavuos with Matan Torah is because in truth the Torah was meant to be given on the 50^th day of the Omer, which is the Shaar Hanun of Bina, and it is due to alternative factors that in the first year the giving of the Torah was delayed until the next day. Accordingly, each year, on the 50^th day of the Omer, the revelations of Matan Torah take effect irrelevant to the day of the month, and therefore even if Shavuos falls on the 5^th or 7^th of Sivan one is to say Zeman Matan Toraseinu. The Rebbe ibid negates his opinion.

[14] Answer of Rebbe to Nitei Gavriel 35:9; Shulchan Menachem 3:25

[15] The reason: As the Holiday of Shavuos is not dependent on the actual counting of the Omer, but rather on the physical occurrence of 50 days. [Likkutei Sichos 3:997]

[16] Igros Kodesh 20:112 and 148; Answer of Rebbe, brought in Nitei Gavriel 35 footnote13

[17] Beir Moshe 7 Elektri 90; Kinyan Torah 5:46; See Mikraeiy Kodesh Pesach 2:63; See Igros Kodesh 26:250 who leaves this matter with a Tzaruch Iyun