Two islands, 3.8 km apart, 21 hours of time difference.
The Diomede Islands sit in the Bering Strait: Big Diomede belongs to Russia, Little Diomede to the USA, 3.8 km apart at their closest. The International Date Line runs between them. Big Diomede is 21 hours ahead of Little Diomede, 20 in summer. Hence "Tomorrow Island" and "Yesterday Island". Little Diomede has 77 residents; Big Diomede only soldiers.
Useful: From the American island you can literally look into tomorrow.
Source: en.wikipedia.org