Coordination devices: daylight savings time, and the international dateline

Daylight savings time is a coordination device that we think about twice a year. The international dateline is another...

The Border That Stole 500 Birthdays
"So how exactly does drawing a line across the Pacific solve this issue? Why isn’t it enough merely to change the date at the stroke of midnight in each successive time zone? Because, if you think about it, that’s logically impossible. With only a single line moving westward across the planet, what exactly is it separating? The same date from itself? No — it is precisely because midnight separates two dates from each other that we need two date lines to separate them, one moving and one fixed.

"Picture that movable date line — the stroke of midnight — racing across the earth at the speed of one time zone an hour. When the clock strikes 12  in the first zone west of the fixed date line, a new date is born in a sliver one time zone wide, stretching from pole to pole. As the hours tick away, that slice grows wider across the Earth’s circumference. The new date races ahead of the sunrise, lighting up the east as the day races west. Inexorably, as the date circles the Earth to rejoin the fixed line, the “new” date becomes the “old” one, to be replaced in turn by the next one as the midnight hour once again crosses the date line"