The Big Diomede and Little Diomede are islands in the middle of the Bering Strait. Although only 4 km apart, they belong to different countries and have a 21-hours time zone difference.
Flat-Earthers noticed the number of polar days is different on both islands during the summer solstice & they used it to discredit science. In reality, the islands are very close to the Arctic circle, & during the June solstice, the sun’s path is almost parallel to the horizon. A tiny change in latitude or height can result in a large change in sunrise & sunset times.