Q. How does the international date line impact on Jewish observance?
A. The problem is that when you cross the date line in one direction you gain a day and if you go the other way you lose a day. This has an effect on when you observe Shabbat or the festivals, as well as on counting the Omer, saying the Psalm of the day, lighting Chanukah candles, counting the days of mourning, or before going to the mikvah.
The verse, “It is a Sabbath to the Lord in all your dwellings” (Lev. 23:3) is understood by Sforno as saying that the date of Shabbat is determined “according to your habitations” – i.e. Shabbat depends on where you live. It is earlier in New Zealand, and later in New York. Rabbi Jacob Emden said in the 18th century that someone who travels from east to west or vice versa should keep Shabbat according to his place of origin and adopt a different Shabbat when he reaches a settled Jewish community.
This rule is endorsed by Rabbi Abraham Eber Hirschowitz, who reports in his Bet Avraham that he faced the problem when he travelled from Sydney to San Francisco a century ago.
Given that the halachic considerations of crossing the date line are complex, air travellers whose flights cross the date line should avoid travelling on a Friday, and should consult a rabbinic authority before travel.