How are the timezones divided?

Answer 1

It would be so easy to figure out the time anywhere in the world if time zone boundaries exactly followed lines of longitude, if every two consecutive time zone boundaries were separated by exactly 15° of longitude, if the time difference between any two zones were always a multiple of a whole hour, if every time zone's offset from Coordinated Universal Time (UTC) were one of the 25 whole numbers from -12 to +12, and if there were no Daylight Saving Time (DST). Welcome to the real world. Each country is free to choose whatever time rules they desire within their borders. Aside from the European Union and the fact that most (but not all) North American countries that observe Daylight Saving Time have chosen to follow the same schedule as the United States for the sake of convenience, there are no worldwide internationally agreed-upon standards for time zones and DST. So first of all, rather than most time zone boundaries coinciding with longitudinal lines they instead coincide with international borderlines. In most countries the time is the same throughout the country. Of course some countries span too many degrees of longitude for that to be practical. Time zone boundaries within those countries also are shaped by politics far more often than by geography. At least most of them have managed not to subdivide individual states or provinces among time zones, but that's certainly not the case with the United States and Canada. There are quite a few places within the US where time zone boundaries are county borderlines, and there are five individual counties in the U.S. that are officially divided between two time zones each. In Canada, there are seven different non-adjacent areas that do not observe Daylight Saving Time.

China is a good example of time zone bounndaries being a lot more than 15° apart. The standard time throughout the country is UTC+8 despite the fact that China spans more than 61° of longitude. As a result, at China's easternmost point sunrise times range from 2:59 AM CNST to 6:52 AM CNST and sunset times range from 3:06 PM CNST to 7:06 PM CNST and at China's westernmost point sunrise times range from 7:39 AM CNST to 10:26 AM CNST and sunset times range from 7:42 PM CNST to 10:37 PM CNST. Russia no longer does annual time changes, but most places in Russia are in a time zone that is about two hours ahead of local mean time.

For some reason there is an unusually large number number of people who think that there are 24 time zones around the world and that every time zone offset is UTC plus one of the whole numbers in the range from -12 to +12. Even if that were true, there are 25 whole numbers in that range, not 24. The truth is that from the 21st or 22nd of March until the 21st or 22nd of September of every year there are 38 time zones around the world, and during the other half of every year the total is 39. Not only are 13 of the 39 offsets not multiples of a whole hour, but three of them are not even multiples of a half hour! The range from -12 to +12 is wrong, too. The real range is -11 to +14. That means that there is one hour per day during which it is three different dates around the world.

In a perfect world I don't think we would have semi-annual clock changes for Daylight Saving Time. In a slightly less than perfect world I think we could manage to consolodate most if not all annual time changes into two weekends per year. The reality is that time changes occur in 17 different weekends during 2014. The total number of times at which a time change occurs at one or more places around the world during 2014 is 69. Palestine deliberately makes their DST schedule different from Israel's; I think they're afraid that someone might otherwise think that they were getting along with Israel. Mexico has two different start dates every year for DST and two different end dates every year. Ten Mexican cities along the U.S. border change on the same dates as the U.S., but the rest of the country follows a different schedule. Three other countries, Egypt, Morocco and Western Sahara, have four time changes per year for a different reason: They begin DST in the spring, then they end DST just before the beginning of Ramadan, then they restart DST after Ramadan ends, then they end DST again in the fall. Most Christian countries that do DST do their time changes during the night between Saturday and Sunday, while most Muslim countries that do DST do their time changes during the night between Thursday and Friday. One country, Iran, starts DST on the same date of every year and ends DST on the same date of every year, no matter what weekdays those dates fall on. And one other thing that you may have wrongly thought was the same everywhere is the amount of by which clocks are adjusted in places that do DST. It is the same almost everywhere: one hour. However, there is one small exception: Australia's Lord Howe Island springs forward and falls back only a half hour.

Answer 2

Time zones are divided based on lines of longitude. The prime meridian, which passes through Greenwich, London, serves as the reference point for dividing the world into time zones. Each time zone covers a 15-degree range of longitude, with neighboring zones differing by one hour. There are a total of 24 time zones, each one hour apart.