If it is 31st December, 2023 in New Zealand, at the same time what will be the date in the Hawaiian Islands and Alaska?

Correct — B, Both have 30 December, 2023. New Zealand lies just west of the International Date Line and keeps one of the earliest clocks in the world, at 12 hours ahead of Greenwich in winter and 13 hours ahead on daylight time, which is what applies in December. Hawaii runs 10 hours behind Greenwich and Alaska 9 hours behind. New Zealand is therefore about 23 hours ahead of Hawaii and about 22 hours ahead of Alaska — very nearly a full day in each case. When New Zealand is somewhere in 31 December 2023, both American territories are still working through 30 December. Take midday in Auckland on 31 December as a check: Honolulu reads about 1 p.m. on 30 December and Anchorage about 2 p.m. on 30 December.

• (a)Both have 01 January, 2024 — Both being on 1 January 2024 would require Hawaii and Alaska to be ahead of New Zealand. They are far behind it — New Zealand sits just west of the date line and they sit east of it, near the western edge of the Americas.
• (c)31 December, 2023 in Alaska and 01 January, 2024 Hawaiian Islands — This splits the two, giving Alaska the same date as New Zealand and Hawaii the next one. Alaska and Hawaii are only an hour apart in December, so they cannot straddle two date boundaries at once; and neither is ahead of New Zealand.
• (d)31 December, 2023 in Hawaiian Islands and 01 January, 2024 in Alaska — This splits them the other way and again puts one of them a day ahead of New Zealand. Both territories lag New Zealand by roughly a full day, so they share the earlier date.

Longitude and time are linked by the Earth's rotation: 360 degrees in 24 hours, so 15 degrees in an hour and 1 degree in four minutes. Places to the east see the Sun earlier and keep later clocks. The International Date Line near the 180-degree meridian is where the calendar date changes, and it bends around island groups so that each country stays on one date. New Zealand sits immediately west of the line and so is among the first places to begin a new day; Hawaii lies well east of it in the mid-Pacific and is among the last.

A rough calculation is quicker and safer here than recalling exact offsets. New Zealand is just west of the date line and Hawaii is far to its east, so the difference must be close to a whole day with New Zealand ahead. That alone eliminates the two options that place either territory on 1 January. The remaining choice is whether Hawaii and Alaska agree, and they do: they are only an hour apart in December, so no date boundary falls between them. This is why New Year is celebrated in New Zealand almost a full day before Hawaii sees it.

• New Zealand is 12 hours ahead of Greenwich in winter and 13 hours ahead on daylight time in December.
• Hawaii is 10 hours behind Greenwich and does not use daylight saving; most of Alaska is 9 hours behind in winter.
• New Zealand is therefore about 23 hours ahead of Hawaii and about 22 hours ahead of Alaska.
• The International Date Line runs near the 180-degree meridian, with deviations to keep countries on one date.
• The Earth rotates 15 degrees of longitude in an hour, that is one degree every four minutes.

• Assuming a place on the American side of the Pacific must be ahead of one in Oceania.
• Forgetting the extra hour of New Zealand daylight time, which applies in December.
• Splitting Hawaii and Alaska across two dates when they are only an hour apart.

A time-zone item that can be answered from the position of the date line alone, without exact offsets.

• practice — not a real PYQ

  If it is 10:00 a.m. Greenwich Mean Time, what is the time at 45 degrees west longitude?

  • (a)7:00 a.m.
  • (b)1:00 p.m.
  • (c)11:00 a.m.
  • (d)4:00 a.m.

  Answer(a) 7:00 a.m. — 45 degrees west is three hours behind Greenwich.

• practice — not a real PYQ

  The International Date Line broadly follows which meridian?

  • (a)0 degrees
  • (b)90 degrees E
  • (c)180 degrees
  • (d)82.5 degrees E

  Answer(c) 180 degrees — with deviations so that countries and island groups stay on one date.