Twice a year, we shift our clocks forward or backward by an hour in a bizarre ritual designed to confuse farm animals and make everyone just a little bit less productive and more irritable for some time in spring and autumn every year. Here in the Central European Time zone (spanning from Spain to Poland), the switch happens at 2:00 winter time: in spring, the clocks skip the hour between 2:00 and 3:00 one night; in autumn, that same hour happens twice. This is also how daylight savings time is implemented across North America.

When people in Britain and Ireland describe this ritual, they sometimes say much the same thing: that the clocks change from 2:00 to 3:00 in spring, and so on. But, fun fact: no! Actually, the clocks are changed at 1:00 GMT, i.e. 2:00 CET. Britain changes her clocks at the exact same moment as its Eastern neighbours, so that clocks in Dover and Calais are always exactly one hour apart, even during the change from summer to winter time and vice versa.

If at this point you’re thinking ‘this sounds like EU harmonization’, you’re right of course, but the history of it is much more surprising than I’d initially thought, and certainly more complex than the Wikipedia article for British Summer Time lets on. Britain and Ireland started using 1:00 GMT rather than 2:00 GMT for the transition to and from summer time in 1981 as a consequence of an EEC council directive, which harmonized the start date and time, but, curiously, not the end date, of summer time.

The end date of summer time wasn’t harmonized until 1995. Until then, most of the EU changed clocks in late September, while Britain and Ireland kept their summer time until late October. This meant that, for many years, Britain, Ireland, and much of the rest of the EEC had about one month per year on the same time zone – one hour ahead of GMT – variously called ‘Central European Time’, ‘British Summer Time’, or ‘Irish Standard Time’, depending on the country. Interestingly enough, this time zone confluence did not include Portugal, which has changed its clocks at the same time as Spain and France the since autumn of 1983 (before Spain or Portugal joined the EEC). In the spring of 1983, however, Portugal (unusually, for that country) changed clocks at 2:00 rather than 1:00. Let's have a closer look at how the time zones of Western Europe played out in the year 1983:

Typically, London and Lisbon are in the same time zone, but in 1983, not only were there 4 weeks where London was on Brussels time, but even an hour in March where London, Lisbon and Brussels were all, somehow, in different time zones.

Now, London and Brussels being in the same time zone isn't that surprising. After all, they're closer to each other than London is to Dublin and much closer than Brussels is to Warsaw, and we're all fairly used to those pairs using the same time.

After I discovered this, I found myself wondering:

What far-apart places that are now firmly in two different time zones once had their clocks set to the same time (however briefly)?

Thankfully, (or tragically,) the Time Zone Database is both comprehensive and machine-readable. Specifically, I looked for pairs of locations which haven't used the same time at any point in the past five years, but which did use the same time at some point in the past – and which were the furthest apart by mean solar time (which should be proportional to difference in longitude).

What follows is a selection of the results.

1942: Japan goes a bit mad

By far the largest anomaly in the history of time zones as far as I can tell was due to Japanese time zone policy during the Pacific War. In 1941, Japan abolished time zone differences across all its territories, including far-flung Micronesian islands like Nauru, standardising all its territories on GMT+9. This time zone reached its greatest extent – from the jungles of Burma to the atolls of Micronesia – with the Japanese invasion of Burma in 1942.

1991: Russia moves West

A few years after the Russian revolution, in 1922, time zones in the Soviet Union were standardized, and the European part of the USSR ended up in GMT+2, the same time zone as Finland and Romania (‘Eastern European Time’). This would have been perfect for St Petersburg (Leningrad), Kiev (Kyiv), Belarus, Moldova and the Baltic republics, and also worked ok for the new capital of Moscow, where the local mean time is near GMT+2:30, but it would have been a bit inconvenient somewhere as far East as Kazan.

Whatever the reason, in 1930, the Soviet Union introduced ‘decree time’, which moved all clocks forward by 1 hour, putting the European part of the USSR on GMT+3. Starting in 1981, the USSR additionally introduced summer time in much the same way as Central and Western Europe.

During the collapse of the Soviet Union, in 1991, most Soviet republics abolished ‘decree time’ and moved their clocks back by one hour. This was certainly an improvement for the Baltic republics, Ukraine, and Moldova, who use EE(S)T (UTC+2/+3) to this day. In Russia, however, this change was apparently less popular, and it was reverted in January 1992.

Nevertheless, during that year, clocks as far East as Ulyanovsk and Kazan could, however briefly, enjoy being set to the same time as clocks as far West as Spain (which, going by solar time, would correspond to an offset of up to almost four hours). As you might imagine, this is, again, a glitch during the transition from summer to winter time.

This curious instance of places separated in longitude by almost ⅙ of a revolution was only possible due to a confluence of three factors:

• Spain has used a time zone that belongs quite far East of its location since Franco's friendship with Hitler,
• many parts of Russia used a time zone that was at home quite a bit further West in 1991, and
• daylight savings time transitions are great at causing confusion.

1992: Portugal gets up in the dark

Unlike Spain, Portugal wasn’t swayed to synchronize its clocks with Berlin in the 1940s, and stayed on Western European Time (i.e. usually the same time as Britain and Ireland) throughout most of the 20th century. From around 1983, when the country was preparing to join the EEC, it has changed between summer and winter time at the same instant as Spain and Central Europe, albeit generally with an hour’s difference year-round.

In 1992, the year of the Maastricht Treaty and some three years before summer time was fully harmonized across the EU, Portugal decided to take its own special step towards European harmonization: they switched to CE(S)T, abolishing the time zone border with Spain.

While Portugal had missed their chance to use the same time as Kazan by a year, the change still put their clocks in reach of anywhere else in UTC+2, which, at the time, included Jordan, Syria and Turkey (all of which now use UTC+3 without any daylight savings time).

In 1993, for example, Syria switched from summer to winter time at 23:00 (winter time) on Friday 24 September, while Portugal changed the following Sunday morning, giving the two countries a full day of using the same time zone. The following spring, Syria changed its clocks several days after the EU (on Thurday 31 March rather than Sunday 27 March).

None of this would last long, of course. In 1996, Portugal switched back to its old time zones, as attested by perhaps my favourite bit of Time Zone Database marginalia:

Martin Bruckmann (1996-02-29) reports via Peter Ilieve that Portugal is reverting to 0:00 by not moving its clocks this spring. The new Prime Minister was fed up with getting up in the dark in the winter.

Honourable mentions

Since I only looked at time zones which haven't coincided recently, I've completely ignored any time zone anomalies that still exist today. With that in mind, here are some honourable mentions which would fit with the theme:

• In the US, the switch from summer to winter time and vice versa happens on the same day, but not at the same instant, so there would be transition glitches at every time zone border in North America, every year.
• China. Just, ... China.