Q. Re- question 1035 traveling east from Beijing to JFK, You mentioned that when you cross any of the datelines during Shabbos, you will revert 24 hours to erev Shabbos after keeping Shabbos some hours. Would you then have to make havdalah?

A. Betzel Hachochmo (4: 83) quoting Ridbaz and others, rules that someone that kept Shabbos and recited havdala already, then travels east and crosses the dateline back into Shabbos, has to keep Shabbos again; and at the end of this second Shabbos day has to recite a second havdal.
However in our case, Horav Shlomo Miller’s Shlit”a opinion is that he does not have to make havdala, since he only entered a Shabbos area according to the view of the Sefer Hayoman and it was only a partial Shabbos.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a