think of it as ordinal: you're shifting left 1 unit, not moving the line, per se.

Still matters because some places want to make sure it's the same day (according to their local reckoning) as it is somewhere else in particular (usually a major trading partner).

Or the next town over.

The other boundaries are a lot straighter.

Imagine what would happen when the International Date Line ends up in the middle of the US.