On Google Maps, there's a line in the middle of the Pacific Ocean. What does it mean and why does it have such an odd shape?
That is the International Date Line. It's roughly on the 180 meridian. It would, as you can imagine, be very challenging for different sides of an island to be on different days, so it originally jiggled around islands. The 180 meridian crosses land in just three places: very far north, very far south, and the island of Taveuni in Fiji. (My blog entry includes pictures of the sign on the meridian.) It is also a challenge for different islands in the same country to be on different days, which causes more jiggling.
Recently, Samoa decided it wanted to be on the same side as New Zealand and Australia, rather than the same side as American Samoa. This caused more jiggling.