clocks go back change 2015
Clocks change back in 2015 on October 25, reverting to GMT Getty

Clocks are going back again, which is great if you're a farmer, terrible if you like leaving work in the daylight. On Sunday, October 25, clocks in the UK will go back an hour from British Summer Time (BST) to Greenwich Mean Time (GMT). So here are some facts about time zones.

There are 24 geographic time zones

The 24 geographical time zones we know today -- one for every hour of the day -- have their origins in 1764, when the English clockmaker John Harrison worked out how to measure accurately a ship's location at sea using the time. But it wasn't until the International Meridian Conference in 1884, in Washington DC, that the international standard, which uses the Greenwich Meridian, was agreed and adopted.

China has a single time zone

China is 9.6 million square miles big. Yet it operates with a single time zone -- Beijing Time -- even though China spans five of the 24 geographical time zones. "In the summer, for instance, it isn't uncommon in Urumqi, Xinjiang's capital, to see people enjoying a beautiful sunset ... at midnight," wrote Matt Schiavenza in The Atlantic. "Or for the sun to rise there in the winter around 10am. In order to accommodate people inconvenienced by the time zone change, shops and restaurants in Xinjiang often adjust their hours — but the effect can still be disorienting for the unaccustomed traveler."

France has the most time zones

When all territories are included, such as former colonies still under French control, France has the most time zones of any country in the world at 12: from French Polynesia in the South Pacific to the Reunion island in the Indian Ocean.

Samoa and Tokelau lost a day

There was no December 30, 2011, in Samoa and Tokelau as the islands recalibrated their time zone for trade reasons. They just skipped straight to December 31 from 29 so they could be closer in time to Australia and New Zealand. "While it's Friday here, it's Saturday in New Zealand, and when we're at church on Sunday, they're already conducting business in Sydney and Brisbane," said Samoan Prime Minister Tuila'epa Sailele Malielegaoi.

Australia's tiny time zone

Outside of the 24 geographical time zones, some countries run their own. In Australia, one time zone -- "Australian Central Western Standard Time" -- serves a few hundred people at most. And it is an odd one. Rather than a whole number, it is UTC+8:45. Yes, that is eight hours and 45 minutes.

"ACWST is observed only in a tiny sliver in the far southeastern corner of Western Australia along the Eyre Highway, extending from just outside of Caiguna to about 50 metres across the South Australia state line to encompass Border Village, for a total length of about 340 kilometres," according to the Twelve Mile Circle website. "Perhaps only a couple hundred people live within the narrow ACWST strip. This of course made it much easier for them to agree upon a standard time.

A couple hundred people can probably come to consensus on just about anything, apparently even the complete departure from a standard time others say should apply to them. That doesn't concern them. They set their clocks as they please. It's such a small population that the government turns a blind eye to it and allows ACWST to continue albeit without official sanction."