The Diomede Islands are two islands in the Arctic Ocean, between Alaska and Siberia. Despite the distance being 4 kilometres, there is a time difference of 21 hours between them. Why?

I’m asking because it’s quite difficult for me to grasp the concept of time differences when the physical distances are so short. I know of the International Date Line, but I’m not sure what it entails. If any nerds would care to enlighten me, I’d appreciate it!

(This question also applies to the Kiribati Island and Howland Island; the time difference is ~26 hours, yet the physical distance is only ~2160 km?)