A few days ago an interesting test failure occurred in the Elasticsearch test suite, highlighting a rare and obscure assumption-violation that’s lain dormant for a number of years. I hesitate to call it a bug, because it’s not really clear what the correct behaviour is here, but there’s definitely a discrepancy between the assumptions recorded in the test suite and the behaviour of the production code.

The discrepancy only (correction) manifests in Newfoundland (specifically, the America/St_Johns IANA timezone) and only for 59 minutes of each of the years from 1987 to 2010 inclusive.

The assumption that we were incorrectly making was, roughly, that the act of extracting the date from a local time is a nondecreasing function. This is pretty much the case in pretty much all other timezones, but in Newfoundland for a while they set the clocks back each autumn at 00:01 local time, meaning that the first minute of the Sunday on which the clocks changed was followed by a reprise of the last 59 minutes of the previous Saturday, then the first minute of the Sunday again, before normal service was resumed.

A consequence of this situation is that there are two local midnights on each of the days when the clocks go back. This is not the only way this can happen, and Elasticsearch’s test suite already checks for this case in the Atlantic/Azores timezone which sets its clocks back by an hour at 01:00 so the first hour of the day of the change is repeated, including midnight, without any overlap between that day and the previous.

Now it’s fairly well-known that essentially all reasonable assumptions about time are wrong so this particular case isn’t that surprising. I tweeted about it seeing as how it wasn’t on either of Noah’s lists.


One of the responses was this one from James Davenport pointing me to the temporal effects of the purchase of Alaska from Russia by the United States of America, which is another interesting curiosity. Since this occurred in 1867 it pre-dated the adoption of time zones, so time was typically defined by local solar time everywhere. The transfer of sovereignty didn’t change the movement of the sun across the sky, so the effect of the purchase on local clocks was nonexistent. The effect on calendars was more complicated. Firstly, Russia sits on the opposite side of the International Date Line from the USA, so the transfer shifted calendars back a day. Secondly, at that time Russia was using the Julian calendar and the USA was using the Gregorian calendar, which was 12 calendar days ahead at the time.

According to the Sacramento Daily Union at 1530 on Friday 18 October 1867 there was a ceremony in Sitka marking the transfer. The preceding sentence is itself a little confusing as the date, appearing as it does in an American newspaper, is presumably given in the Gregorian calendar, and also presumably given according to the American side of the International Date Line. It’s not wholly clear, but this is how the IANA Timezone database interprets it at least: taking the time of the transition to have occurred at the time of the ceremony then then in the surrounding seconds the local time at Sitka would have looked like this:

Saturday 1867-10-07 15:29:58
Saturday 1867-10-07 15:29:59
Friday   1867-10-18 15:30:00
Friday   1867-10-18 15:30:01
Friday   1867-10-18 15:30:02

This is wrong for a couple of reasons. Firstly, I’m using ISO 8601 date formatting, but ISO 8601 prescribes the Gregorian calendar, so those first few days should really be written 1867-10-19, or else something like 7 октября 1867 г. that isn’t in an ISO 8601 format. Written like that, this is another counterexample to the assumption that date truncation is nondecreasing.

It’s also wrong because this description of the transition is very precise but very inaccurate. Time and date weren’t terribly well-documented or communicated back in 1867, and governments took even less interest in defining them precisely than they do now. I don’t know that there’s any evidence that the locals of Sitka set their calendars back to Friday at the transition ceremony, and it seems unlikely to me that anyone did. There’s an account of someone who kept on the Russian calendar for another couple of days, and then celebrated two Sundays in a row, so perhaps it’d be more accurate to have the transition at midnight on 1867-10-21, although the account describes the second Sunday as “our Monday” so perhaps they didn’t change their calendar even then. Changing everywhere at local midnight would give a nondecreasing date truncation function again, but would mean that different locations in Alaska would change at different times since local time was solar time back then.

Communication in Alaska at that time was pretty poor, so it seems likely that anyone outside of Sikta was unaware of the transition for quite a while yet. Frankly, if you’re trying to interpret temporal data from Alaska in 1867, you’re going to have to work out what time zone and calendar were in use yourself. It’s almost certainly not in any time zone described by the IANA database.

Samoa and Tokelau

There are various other examples of countries moving across the International Date Line that pre-date the adoption of timezones. In many cases (e.g. Pacific/Pago_Pago, Pacific/Apia) the IANA database records these shifts as occurring at local midnight, giving a simple 48-hour day with no overlaps with the neighbouring days.

A more recent example of a shift across the International Date Line is the case of Samoa and Tokelau who changed sides because of the effect on their calendar. The change, in the opposite direction from Alaska in 1867, took place at local midnight, skipping over Friday 2011-12-30 entirely. The date truncation function is nondecreasing here, but this is a nice source of other counterexamples.


Addendum 2018-01-04. I was interested in how rare a failure this is so I went through the IANA database (version 2017c) looking for similar situations of overlapping days. The only regular ones since 1950 are the clocks going back in America/St_Johns (Newfoundland, 1987-2010), America/Goose_Bay (Labrador, 1987-2010) and America/Moncton (New Brunswick, 1993-2006) as described above. As a further bonus, in America/St_Johns in 1988 the clocks actually went back 2 hours at 00:01. Eastern Canada is a special place indeed.

There was also a one-off shift in Antarctica/Casey by three hours from UTC+11 to UTC+8 at 2010-03-05 02:00 local time which took it back to 2010-03-04 for an extra hour.

Prior to 1950 there’s a bunch more examples of overlapping days, mostly with strange offsets due to the more common use of solar time in that era. One doozy that particularly stands out is the case of America/Phoenix who apparently set the clocks back by an hour at 1944-01-01 00:01, causing an overlap between the years of 1943 and 1944. I’m not sure this actually happened, because the quoted source for this information doesn’t give a specific time, and the clocks have generally gone back at 02:00 in the USA for as long as daylight saving time has existed, but it’s something you’ll need to handle if you are using the IANA database as your source of truth.