The time every city's clock showed something different
Time is a difficult concept. The longer you think about it, the more complicate it gets. Fortunately, you don’t really need to think about it either – it’s just like maths in that respect ;-).
Nevertheless, I was surprised to learn that it wasn’t until 1909 that a uniform time was used in the Netherlands: 19 minutes and 32.13 seconds ahead of Greenwich Mean Time. In 1937, this was simplified to 20 minutes (GMT +00:20), and when Germany occupied The Netherlands in May 1940, Central European Time (GMT +01:00) was established as the official Dutch time. This finding made me want to investigate the concept of time and time zones further.
Local mean solar time
Before clocks were in use, the time was measured based on the Sun's position in the sky using sundials. A sundial is a device that tells the time of day by the apparent position of the Sun in the sky. An apparent or “real” solar day is the time that passes between the Sun’s highest position today and her highest position tomorrow. Because the Earth’s orbit is elliptic and not circular, and because the Earth’s axis is tilted, apparent solar days differ in length through the year. This variation, however, is subtle: there is only a one minute difference between the longest (during the solstices in June and December) and the shortest (during the equinoxes in March and September) solar day.
In the beginning of the 19th century, well-regulated mechanical clocks became widespread. Because it is rather impossible to construct clocks based on days that vary in length, mean solar time was established. This time is based on the average length of a day: 24 hours.*
Greenwich Mean Time
Because the sun’s position is dependent of the position of the observer on earth, each city began to use some local mean solar time. One of those is Greenwich Mean Time (GMT), which was established in 1675 when the British Royal Observatory was built in Greenwich. Noon GMT is the moment at which the sun reaches its highest point in the sky over Greenwich. Due to the day length variations as mentioned above, this moment rarely is exactly at 12:00:00 GMT, with variations of up to 16 minutes. The establishment of GMT was of great help for navigators at sea, who now had a standard time to refer to when each city in Europe kept its own local time.
Things got more difficult when rail transportation improved. The various local times made it extremely complicated to make time tables, since every geographical longitude meant a difference of four minutes. So for the Netherlands, when it was 12:00 in Amsterdam, it was 11:58 in Rotterdam and 12:08 in Enschede!
This made it necessary to agree on a standard time. The first standard time was adopted on December 1, 1847, in Great Britain by railway companies, soon known as Railway Time. Initially this only meant that the time of clocks on railway stations was agreed on to be in GMT, but these still differed from the times as shown on the clock on, say, the town hall. People had to adjust their watches when they were travelling. This situation lasted until 1880, when GMT was made Britain’s legal time.
The establishment of time zones
Almost the entire Earth was divided in standard time zones. However, these were mostly based on the location of a local astronomical observatory in a country. In the Netherlands, for example, Amsterdam time was set as the standard time in 1909. This was 19m32s ahead of Greenwich (simplified to 20m0s in 1937).
In 1884, at the International Meridian Conference, the Greenwich meridian was defined as the zero degrees meridian and so GMT became the standard time to which all other time zones could refer. In 1960, GMT was being replaced by UTC, which is basically the same, but UTC is more precise.
Within the next decades, countries all over the world adopted a time with an offset from GMT that is a multiple of half an hour, or, in most cases, an hour, like Central European Time (CET, UTC +01:00) or Pacific Time (UTC -08:00). The Netherlands were very late in adopting CET and did not even do it voluntarily, but had to adopt the time zone when Germany occupied the country in May 1940.
*Actually it takes the Earth 23 hours, 56 minutes and 4.09 seconds. A solar year thus takes about 365 days, 5 hours and 49 minutes. The missing time of about 6 hours a year is corrected by having a leap year every 4 years by extending February to 29 days. Without the added day, the seasons would move back in the calendar. But adding a day every 4 years would add 44 minutes to the calendar and a shift would still occur! Therefore, the leap year is being skipped three times in 400 years to make up for that.