Time zones
The idea is a simple one. We want 12.00 noon to correspond roughly to when the sun is at its highest in the sky. To achieve this, we divide the world up into 24 time zones, each one an hour different from the next. France and England fall into adjacent time zones and so are one hour apart. Since the earth rotates anticlockwise (looking from “above”), the sun rises in France before it rises in England, and the sun will reach its zenith over France before it does so in England. Thus the time in France will need to be one hour ahead of England. However, everyone will agree perfectly on the time within a time zone.
The sun will actually reach its highest point first at the eastern end of a given time zone, so the idea of noon corresponding precisely to the sun’s highest point is unfortunately not generally the case.
Since it would be rather inconvenient for a small country to bridge more than one time zone, the time zone boundaries tend to align with national borders. This is what we get:
Officially the time in the far east of China (Shanghai, say) is the same as it is at China’s western border! The USA is more pragmatic and spans 4 main time zones.
GMT
We can specify a time zone by reference to Greenwich Mean Time (ie English time – at least in the winter…). Greenwich, situated along the Greenwich Meridian (the 0 degrees longitude line) is supposed to be pretty much the point at which the sun is the highest at noon (putting to one side variations over the year and the effect of the axial tilt of the earth!). Argentina, for example, is at “GMT – 3 hours”, “New York is “GMT-5” (Eastern Standard Time) and Tokyo is at GMT + 9. The acronym “UTC” these days is now preferred to “GMT”.
The date line
If we point to London on the globe, and follow the earth around from west to east, adding an hour for each time zone we cross, we’ll finally end up back where we started, but a day later! To avoid this paradox we fix an “International Date line” (IDL), which we’ve decided to lie along 180 degrees longitude. You can see it on the far right in the map above. If you cross it going east, you go back a day. And if you cross it going west, you move forward a day. If it’s Monday in Japan, it’s Sunday in the US. And when it’s 3am Monday morning in New Zealand, it’s 5am Sunday morning in Hawaii – almost a whole day’s difference even though the islands are just two time zones away!
I remember that a few years ago the people of Samoa decided it would be more convenient to share the same day as New Zealand and Australia with whom they work closely. They therefore decided to pass to the other side of the data line, and had to wind forward their clocks by 23 hours.
Daylight Saving Time
Leaving it there would be too easy. We have to talk about “daylight saving”! In the summertime, we love to make the most of the long days. The sun is shining when we get up, and the sun goes down when we’re still enjoying a BBQ in the garden. Someone had the bright idea to shift the day forward – ie wind our clocks one hour later so that rather than the sun setting at say 8pm it would set at 9pm, and we could enjoy more daylight in the evening. The original motivation for “daylight saving time” was indeed to reduce energy consumption (less need for artificial lighting and heating in the evenings) and also to improve public safety with fewer traffic accidents. There’s no general rule but in the northern hemisphere “daylight saving” tends to start sometime in March and finishes in October or November, depending on the country and the year.
To be clear, “standard time” (winter time) is pretty close to “solar time” in which noon corresponds roughly to when the sun is at its highest. Daylight saving time (summer time) shifts the clocks forward an hour, so solar noon actually occurs around 1pm rather than 12,00pm. In the winter, the time in England is GMT (or “UTC”), but in the summer it’s “BST” (British Summer Time), being GMT + 1. Without DST, the sun might rise at 4am in the summer when most people are sleeping, and it might set at 8pm. But with DST the sun would rise at 5am (when most people are still sleeping) and set at 9pm.
Most English people remember the rule with “Spring forward, Fall back”. Unfortunately, I have a mental block with the words “forward” and “back” as they relate to time keeping. After all, don’t we say “bring the meeting forward” to mean reschedule it earlier? So I just remember it as wintertime being the benchmark, but during the summer months we want to make the most of the long evenings and so we add on an hour.
You can see from the map below which countries implement “daylight saving time”:
There’s less call for daylight saving near the equator where there isn’t so much variation in the length of the daylight hours across the seasons.
The European Commission proposed to end the biannual clock change across the EU back in 2018, but Covid derailed it, and I believe the issue has since been shelved so the EU still exercises daylight saving.
Why not implement “daylight saving” all the year round?
If it’s such a great idea, why not stick with it all year long? After all, we are awake more hours after noon than before noon and could enjoy longer evenings throughout the year. The problem is that the days are so short in the winter that if we continued to shift the time forward, we would be waking up and going to school or work in the dark. We have to accept that we finish work at nighttime, but to start the day in the dark too would be too much!