Why do vehicles travelling westwards deduct a day when the cross the international date line?

Answer 1

-- First of all . . . the International Dateline doesn't cross any land except on
Antarctica. So a vehicle, as in 'wheeled landcraft', that's crossing it, has a more
immediate problem than figuring out what day it is. In order to deal with this
question, we'll consider ships and aircraft that cross the line.

-- The few that actually 'deduct' a day do it because their passengers and crew
have been misled and are confused. The sad result is that when they reach their
destination, their calendars disagree with those of the locals by two days.

-- The correct and appropriate adjustment when crossing the IDL in the westward
direction is to ADD a day, i.e. tear a page off of one's calendar, as if one had instantly
aged by one extra day.