Non-24-hour sunset-based sabbaths

Someone once told me sabbath is a sacred 24-hour period, but considering that they observed sabbath from sunset to sunset every seventh day, I can think of three ways that assertion is not quite accurate

Changing day length

Firstly, the time between sunsets is not exactly 24 hours. Between the winter solstice and summer solstice, days are getting longer, so each sunset is a little later than the one before it. The reverse happens from the summer solstice to the winter solstice, when days are shortening. For anyone observing sabbath from sunset to sunset, winter and spring sabbaths will be slightly longer than 24 hours, while summer and autumn sabbaths will be slightly shorter.

In practice, the difference is negligible. The biggest difference seems to occur near the antarctic circle in late November, with sunset being about 4 minutes earlier than the previous day. In a more temperate latitude like where I live, the greatest difference is about 2 minutes. At such small differences, atmospheric effects and uneven terrain probably affect sunset more than the changing length of the day.

Timezones

If sabbath begins at sunset, it does not begin at the same time everywhere in the world. Sunset takes 24 hours to circle the globe, so for 24 hours after sabbath begins at the International Date Line in the central Pacific, it has yet to begin just east of there. When sabbath finally does begin there, it soon ends just to the west, and spends another 24 hours ending around the world. Thus, if sabbath is demarked by sunset or any time on a clock, it is sabbath somewhere in the world for 48 hours.

The arctic and antarctic

North of the arctic circle and south of the antarctic circle, there are long periods near the solstices when the sun does not set. Utqiagvik, Alaska experiences an uninterrupted polar day for 84 days and polar night for 68 days, a total of 252 days, almost 70% of the year. If sabbath observance is based on sunset, then most of the year in such places either has no sabbath, or experiences up to two very long sabbaths.

Additionally, anyone who travels to the arctic or antarctic near a solstice will experience fewer sunsets than they would have at home. If they observe every seventh sunset as the start of sabbath, they'll return home to observe a different day of the week than they did when they left. Thus, sunset-based sabbath observance has path dependence.