Every day has 48 hours. Our system of time measurement is based on the Earth being in one spot relative to the Sun. The calendar rotates around this system.
Now, if we go East as far as we can, without putting ourselves at risk as coming from the West, we find ourselves at a place that brings in the new day: when this point, the Earth, and the Sun are in a line.
Then, go West as far as one can, without being at risk as coming from the East. Our first place of rest reaches the end of the day in question; but we are 24 hours before-hand, and only now start it.