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.