The International Date Line

A traveller going eastwards gains time from Greenwich until he reaches the meridian 180 degrees east when he will be 12 hours ahead of G.M.T. Similarly in going westwards, he loses 12 hours when he reaches 180 degrees west. There is thus a total difference of 24 hours or a whole day between the two sides of the 180 degrees meridian. This is the International Date Line where the date changes by exactly one day when it is crossed. A traveller crossing the date line from east to west loses a whole day (because of the loss in time he has made); and while crossing the date line from west to east he gains a day (because of the gain in time he encountered). Thus when it is midnight, Friday on the Asiatic side, by crossing the line eastwards, he gains a day; it will be midnight Thursday on the American side, i.e. he experiences the same calender date twice! When Magellan's ship eventually arrived home in Spain in 1522 after circumnavigating the world from the Atlantic Ocean to the Pacific Ocean and westwards across the International Date Line, the crew knew nothing about adding a day for the one they had missed. They thought they had arrived on the 5 September. They were shocked to be told that the date was 6 September. A modern aircraft leaving Wellington at 5.00 p.m. on Friday reaches Hawaii, 6601 km away, at 2.00 p.m. the same Friday. The same aircraft on its return journey from Hawaii at 11.00 a.m. on Sunday. Can you explain this?
The International Date Line in the mid-Pacific curves from the normal 180 degrees meridian at the Bering Strait, Tonga and other islands to prevent confusion of day and date in some of the island groups that are cut through by the meridian. Some of them keep Asiatic or New Zealand standard time, others follow the American date and time. To find local time in two places on opposite sides of the International Date Line remember that crossing the 'line' going eastwards a whole day is gained. Crossing the 'line' going westwards a whole day is lost.
Example. If someone in Tokyo (time zone 135 degrees east) telephones a friend in Vancouver on 4 December at 10.00 a.m. what time will his friend receive the call in Vancouver (time zone 120 degrees west)?
1. Longitude difference 180' - 135'E = 45'; 180' - 120'W = 60'; 45' + 60' = 105'
2. Therefore time difference = 105/15 = 7 hours.
3. Vancouver is east of Tokyo, therefore time goes on 7 hours.
4. Going eastwards across I.D.L. a whole day is gained. Therefore the time in Vancouver is 5.00 p.m. on 3 December.