Do we ever know that???
Tomorrow Island (Big Diomede) and Yesterday Island (Little Diomede).
The Diomede islands are just three miles apart but the bigger island is almost one day ahead of its smaller neighbour (21 hours) because they sit on either side of the International Date Line which passes through the Pacific Ocean and marks the boundary between one calendar day and the next. They sit in the Bering Strait between mainland Alaska and Siberia
Big Diomede is located on the Russian side while Little Diomede is on the US side. The ice bridge that forms between the two islands in winter makes it possible, although illegal, to walk the short distance between them and 'travel through time'.