VICTOR HABBICK VISIONS/Science Photo Library/Alamy
Eric Kvaalen
Les Essarts-le-Roi, France
Both are possible. If you start in the time zone GMT+12 (in the sea east of New Zealand, say), when it is 1130 GMT on Sunday morning in London, it will be 2330 on Sunday where you are. You can then move a bit east and be in the time zone GMT-11, where it will be 0030 on Sunday. You can stay there in Sunday for another 23.5 hours, making 47 hours passed in Sunday.
If you had started at 2130 in the Line Islands that belong to Kiribati, where the time zone is GMT+14, and a few hours later moved west through the GMT+13 zone of Kiribati, and then gone south to the area east of New Zealand and finally east to the GMT-11 zone, you could stay in Sunday for 49 hours.
If, on the other hand, you start in the GMT-11 zone around Kingman Reef at 1030 GMT (Sunday in London), it will be Saturday at 2330. You can immediately go east into the GMT+14 zone of the Line Islands, where it will be 0030 on Monday. You will have skipped Sunday.
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In any country that adjusts its clocks in spring and autumn, the day the clocks go back will be longer than 24 hours
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Let’s say there’s a man named Odd Oddson who flies around those parts of the Pacific. He could start on the first day of the month near the Kingman Reef at 2330 and cross to the Line Islands, where it is the 3rd of the month. He later goes through the GMT+13 zone of Kiribati and gets to the New Zealand area by 2330 of the 3rd. He then goes east to where it is 0030 on the 3rd, and circles around to the east of the Line Islands back to his starting point. In this way, Odd Oddson can always be living in an odd day of the month.
Garry Smith
St Albans, Hertfordshire, UK
Most of us will have experienced a day of more than 24 hours. In any country that adjusts its clocks in spring and autumn, the day they go back – e.g. the last Sunday in October in northern Europe – will be longer than 24 hours.
Now let’s think about the International Date Line (IDL) in the Pacific Ocean. Let us first assume that the IDL is a straight north-south meridian, and that the territories on either side of it are well-behaved, meaning they are never more than 12 hours ahead of or behind GMT or Coordinated Universal Time (UTC).
If you step over it eastwards at precisely midnight, local time – and if we assume this takes no time at all – then you are crossing from the end of a day, west of the line, into the very beginning of that same day, east of the line. You can now stay in that day until you and the midnight line next cross, which could be any time up to 24 hours later. This time can be added to the time you spent in that day before you stepped over the IDL.
If the IDL were straight and the territories near it were well-behaved, then you wouldn’t be able to skip a date. No matter where you were in the world – including on the IDL – if you crossed the midnight line westwards, you would simply be stepping into the next day.
There are territories – Howland Island and Baker Island, east of the IDL – which have local time 12 hours behind UTC. However, the IDL isn’t a straight line, instead sweeping to the east of Kiribati, which itself is well to the east of Howland and Baker islands. Kiribati’s Line Islands have a local time of UTC+14, while Hawaii and French Polynesia, on a similar longitude, have a local time of UTC-10.
This means there is a 26-hour difference between the territories and, between 1000 and 1200 UTC every day, there are three different days on Earth. For example, Tuesday 1115 UTC is Monday 2315 on Howland Island and Wednesday 0115 in the Line Islands. If, during that 2-hour window, you could quickly hop eastwards from Howland Island or Baker Island – uninhabited atolls – to the Line Islands, a distance of about 1400 kilometres, then you would be skipping a calendar day.
This is only possible, of course, because the IDL, calendar dates, names of days and local times are all artificial, human constructs.
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