UTC time with the most uniform date
February 20, 2020 11:27 AM   Subscribe

Thanks to time zone weirdness, apparently there is no time on the clock at which the entire world is officially on the same date. So at what time does as much of the world as possible share the same date?

This question has no practical purpose, I just think it's interesting. I've consulted Google and done some of my own investigation but it seems there is enough weirdness with time zones that the answer is not straightforward. So if you had to pick a time of day on which as much of the world as possible is on the same date, what time would that be?

I'm purposely leaving this a little vague, because I'm not sure whether "most" should be by land area or by population, or whether those answers would be different. There's also probably details that I'm not thinking of so I don't want to accidentally define away those issues.
posted by Tehhund to Grab Bag (10 answers total) 1 user marked this as a favorite
1am California time is 6pm Tokyo time on the same date, so I imagine that covers most of the landmass and population figures combined?
posted by Grither at 11:38 AM on February 20, 2020

Make it 1:01am (9:01am UTC) and you've got alaska time zone, too. The Pacific is rather large and empty.
posted by Grither at 11:40 AM on February 20, 2020 [1 favorite]

Checking against the timezone data on my computer, the only population centers I could find that were on a different date when it was 12:01am Hawaii Standard Time were:

Pago Pago: 23:01 the previous day
Kiritimati: 00:01 the next day
Niue: 23:01 the previous day
Midway: 23:01 the previous day
Apia: 00:01 the next day

at 12:01am in Hawaii it is 23:01pm in New Zealand, so moving an hour in either direction will stop dropping off millions of people.
posted by 1970s Antihero at 12:08 PM on February 20, 2020 [6 favorites]

Because the International Date Line roughly follows the 180° line of longitude in the Pacific, which is by definition on the other side of the world from 0°, or the prime meridian that goes through the UK (among many other countries), it's going to be within several hours of 12:00 UTC time, i.e. noon in London.

When it is noon in London that means it is midnight on the other side of the world at 180°, so going east from 180° to London (i.e. passing through the Americas) it will go from midnight to noon on that same day; and then going east from London back to 180° (i.e. passing through Africa and Asia) will go from noon to the following midnight on that same day.

Of course, this is a broad simplification (as the OP has clearly realized), because of factors like the fact that the International Date Line veers widely from the 180° line in parts, which is how you get time zones like UTC+14, and the effects of summer time/daylight saving time in the northern and southern hemispheres.

It would be interesting to see if the answer differs in the northern vs. southern summers, though I'm unfortunately not in a position to answer this more authoritatively than this!
posted by andrewesque at 12:23 PM on February 20, 2020 [3 favorites]

Funny, I was looking at something similar recently. You may have already encountered this on Wikipedia's time zone list page:
Some zones that are north/south of each other in the mid Pacific differ by 24 hours in time – they have the same time of day but dates that are one day apart. The two extreme time zones on Earth (both in the mid Pacific) differ by 26 hours.
Going from there, you would either need to lop off the two "earliest" hours of time zones or the two "latest" to get down from 26 hours to 24 hours. The former would include:
UTC+12:45, M

New Zealand: Chatham Islands

UTC+13:00, M

Kiribati: Phoenix Islands
New Zealand: Tokelau

UTC+14:00, M

Kiribati: Line Islands
The latter would include:
UTC−12:00, Y

United States Minor Outlying Islands: Baker Island & Howland Island

UTC−11:00, X

United States: American Samoa
United States Minor Outlying Islands: Jarvis Island, Kingman Reef, Midway Atoll, Palmyra Atoll
New Zealand: Niue
posted by Conrad Cornelius o'Donald o'Dell at 12:24 PM on February 20, 2020 [2 favorites]

Nobody lives in UTC-12 or in the US Minor Outlying Islands. In the southern winter, at 10:30 UTC it's 00:30 in Hawaii and 23:30 in Samoa. You only lose American Samoa (55k people), Niue (2k), and the Line Islands (9k) - so it's the same day for everyone except those 66k or so people. If you're willing to say that two times 24 hours apart are the same day, then at the stroke of midnight in American Samoa and Niue (11:00 UTC) it's the same day for everyone except the Line Islands.

The southern summer is a bit trickier...
posted by madcaptenor at 12:37 PM on February 20, 2020 [3 favorites]

This is also complicated by summer time changes in the northern and southern hemispheres, which means accounting for New Zealand, which is currently at UTC+13, but UTC+12 in winter. So 2359 in Auckland right now is 0059 in Hawaii.

(Here's a place to do the comparisons.)
posted by holgate at 12:40 PM on February 20, 2020 [2 favorites]

Response by poster: 9:01 am UTC. Got it.
posted by Tehhund at 2:31 PM on February 20, 2020

Best answer: 9:01am UTC doesn't include Hawaii, but 10:01am UTC does.
posted by 1970s Antihero at 6:35 AM on February 21, 2020 [2 favorites]

Response by poster: 10:01 AM UTC - got it.
posted by Tehhund at 8:05 AM on February 21, 2020

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