A ship sailing from the eastern extremity of the Aleutian Islands to Dutch harbour crosses 180° meridian at 23:30 hrs on January 1, 1999. What time and date will be recorded by the captain of the ship in his diary after a one-hour journey from the point of crossing the meridian?

Answer: (b) January 2, 00:30 hrs

The key gives January 2, 00:30 — one hour later with the date moving on at midnight.

The idea

The International Date Line keeps the calendar consistent when clocks have been followed around Earth. A genuine crossing changes the date, but simply crossing 180° longitude need not do so. Near the Aleutian Islands the date line bends away from that meridian to keep the islands on the same calendar side.

Identify the ship’s actual route → Check whether it crosses the date line or only 180° → Apply a date change only for a genuine date-line crossing → Add the elapsed journey time → Advance the date if the clock passes midnight

Why the others are wrong

(a) Crossing eastward sets the date back a day.

(c) Crossing westward adds a day.

The trap

Direction decides everything: west → add a day, east → subtract a day.

Also know

At a genuine date-line crossing, westward travel adds a calendar day and eastward travel subtracts one. Civil time zones and the date line are political arrangements, not exact longitude lines everywhere. The Prime Meridian sets the longitude reference; the date line handles the calendar change on the other side of Earth.