shabbat - When does a day start/end in the far North/South?

As I understand it, a day in the Jewish calendar is from sunset to sunset. How is this handled if you are living so far north or south that you have long periods of daylight or darkness?

Answer

This affects when Shabbat and yom tov start and end and when you can perform time-bound mitzvot. There are various opinions (some collected here), so this is something you need to consult your rabbi on. Opinions cited there include: use the times for your home city (if you're visiting); use 6PM; use the point when the sun is at its lowest in the sky.

I once learned that you use the time of the nearest Jewish community that has daily sunrise/sunset, but I can't find support for that now.