Why of all units is Time standard?

From a scientific perspective, all units ARE standard, under the International System of Units. There are seven base units, including metre for length, kilogram for mass and second for time. The use of yards, or pounds, are a local eccentricity.

As someone else pointed out, railroads were the impetus to get away from local time based on local noon. Lack of a standard time was responsible for many accidents. Harrison is also mentioned -- the connection between time and geographical location made the desire for precise time to have long preceded the ability to obtain it reliably.

Interestingly, the very concept of an abstract and unvarying unit of time has not always existed (perhaps) -- an inventor in ancient Greece devised a clock that was used to measure time in court (things like how long someone could speak) and he spent a lot of effort on making the time vary depending upon the season since they saw hours not as a fixed length but rather a division of the daylight hours into 12, so much shorter period in winter. I do not know if we can conclude from this that the ancient Greeks did not have the concept of a fixed hour but it seems possible and I would wonder when an hour that was the same in summer and winter came about. I would guess that by the 1600s (if not well before) an abstract time not tied to the sun existed since, for example, people were interested in the speed of sound and light and it would be silly to talk about the speed in "winter seconds" or whatever.

One of the design principals of the Metric System is that the units should be based on universal natural phenomena, not random human artifacts (or average grains of corn, etc.) -- so the kilogram was originally the weight of a cube of water at a certain temperature sized at a certain fraction of a meter, the meter was a certain fraction of the earth's circumference, and temperature was calibrated to observable state changes in water.

As time went on, some of the units were redefined to match greater technological precision in measurements and a realization that some natural phenomena aren't really universal, or change over time.

I don't know for a fact, but I presume that seconds were considered to be a reasonable natural-phenomena based measurement back in the early 1800s as it was based on the rotation of the earth. As with other units, the definition of a second has changed over the years to match increased precision and the knowledge that the earth's rotation period is variable.

See Wikipedia's article on the Metric System: https://en.wikipedia.org/wiki/Metric_system

There is a simple connection between litres, metres and grammes. A litre is 1/1000 of a metre cubed. A gramme is 1/1000 of the mass of a litre of water.

Originally it would have been possible to define a metre as being equal to a yard. The corresponding litre (one yard cubed / 1000) would not be a decimal multiple of the pint or the gallon. The corresponding gramme would not be an decimal multiple of the ounce or pound. If we want to maintain the nice relationship between the length, mass and volume at least two of the old units needs to be replaced. The French decided to replace all three. The metre was defined as being 1/10,000,000 of the distance from the North Pole to the equator.

At the time that the metric system was introduced there was no simple way to connect time to mass/volume/distance, so time measurements were dealt with separately - the old second was retained. If we were to reinvent the metric system today we could possibly define the newsecond as being the time that it takes light to travel 100,000,000 metres.

Notes)

1) In the Imperial system there is actually a decimal relationship between mass and volume - a gallon is the volume of 10 pounds of water. This does not work in the American system.

2) the standard litre is actually 28 parts per million larger than it should be due to an error in the manufacture of the standard litre container.

Of course there have been many systems of time, as elaborated in some of the great other answers.

There are several differences between time and systems like weight or length:

• Weight and length are required by everyone, so everyone has to come up with a system. Can I carry this? How big is this? Is this animal in range of my spear? How valuable is this in trade? These are questions humans had tens of thousands of years ago. With the rising importance of trade a couple thousand years ago, there is soon a requirement for very large and very small measurements. Trade distances were short in the beginning, and converting units was easy, so a lot of different systems existed in parallel.

• Time is required by everyone, up to a certain precision. A day, many days, a season, morning, noon, evening, these are enough for most people. For a long time measuring time on a smaller scale was much harder than measuring length or weight, so most people simply didn't bother. It's easier to standardize a system if fewer people use it.

• Converting weights and lengths requires one multiplication. Converting calendars is near rocket science (e.g. leap years in one calendar but not the other), which makes different calendars an impediment to international trade.

So standardizing time was simpler (less competing systems, less people using time), and the benefit of standardizing time was greater, so it happened first. Specifically about the American units: Time was already standardized in Europe by the time the Americas were colonized, while weight and length were not.

China had its own time measurement, defined by time it takes to burn a stick of incense, boil a pot of water, cook a bowl of rice, etc. It was abandoned because of its inaccuracy.

In their original zeal for things decimal, when the French originally adopted the metric system after the Revolution, they also adopted a new system of time keeping called French Revolutionary Time to go along with the French Republican calendar.

French Revolutionary time divided the period of the day from midnight to midnight into 10 hours, each hour being divided into 100 decimal minutes each containing 100 decimal seconds. There are still extant some clocks made to keep time in this system.

Decimal Time

The description of the French Republican calendar is best left to those interested in such historical minutiae. The Republican calendar was abolished in 1805 after the French themselves found it too complicated to follow.

Republican Calendar

Decimal time had an even briefer run, being in mandatory use only between September 1794 and April 1795, although some decimal municipal clocks and personal watches were made to keep decimal time.

The mathematician Laplace had a decimal watch made for him which kept time in fractions of a day, and he kept all his time in these units, which is reportedly why astronomers presently reckon time using fractions of a day.

Time is not standard at all. Historically, there have been many different systems of measure.

The year has been measured in lunar and solar months. For instance the Muslim world uses a lunar calendar starting in the 600s for religious purposes. In Roman times, the calendar was also revised several times.

The initial date is now widely accepted as the supposed birth of Christ, but before Christianity popularized this fashion, many nations had their own reckonings based on things such as important local events or reigns of monarchs.

The subdivisions of the day evolved by culture and time also. Before modern clocks were commonplace, there were several systems for dividing the day, sometimes with named hours. I believe in some cases the day would be divided into the same number of hours regardless of season, leading to an unstable measurement system ("short hours" in winter).

Even with our modern system, there have been alternatives proposed. Famously Revolutionary France attempted to use 10-hour clocks (some survive to this day).

The spread of unit systems is closely tied to the spread of measurement technology. Typically, devices for measuring time are more complex and difficult to manufacture than length or weight. Consider for instance that the ruler remains the tool of choice to measure length even today, the balance scale is an acceptable method to ascertain weight, but most people would laugh at the idea of substituting an hourglass or sundial for their digital watch (so much so that of all these quantities, we chose to carry on our person a device for measuring time only). Thus it was harder for alternative systems of timekeeping to emerge after the spread of modern clocks, because most societies simply adopted someone else's technology (and unit system along with it) as opposed to inventing their own.

In contrast, making a ruler is not exactly a great intellectual feat. As such, by the time more advanced weight measuring technology became common place, everybody and their brother had their own "traditional" ruler system. An example that remains today is the United States - which to this day uses an arguably inferior system, despite some notorious accidents occurring as a result, because the entrenchment in society and industry is greater than the pressure of adopting foreign technology.

Why does everyone use seconds/minutes/hours today? These units were adopted in the west in ancient times, by the Greeks, who borrowed it from their middle eastern neighbors, the Babylonians, who had used it before them.

It's all because time-keeping, with seconds, minutes, and hours, originates with astronomical observation, and the art and science are ancient: the 60 seconds dates to at least the late Babylonian period, and was taken up by the Greeks.

Tracing back further, the Babylonian's defined the circle as 360 degrees, as did the Akkadians, and the Sumerians before them: The 360-degree circle is 4400 years old. The usual theory is that 360 is close to the number of days in a year, and as a multiple of 60, it has many exact divisors: 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, 24, 30, 36, 40, 45, 60, 72, 90, 120, 180, and 360. This makes 24 exact divisors!

As the stars are the great clock, astronomers lead the way in time-keeping, and their systems were useful also for navigation. In the West, the continuity of history almost guaranteed the primacy of a single system; the march of technology and armies carried it to the rest of the world during the last few hundred years. Today it is enshrined in international standards. For example, China converted to the western system in 1645.

See Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day?, in Scientific American.

All through history, people needed fair, repeatable measurement of physical quantities. How much grain am I buying? How much land do you have (so it can be taxed)? How much flour and water do I use for my bread? Since most trade was local, they used whatever system they came up with. So long as the people nearby understood it, and it was based on some objective measure, it was good enough. And they built tools around those systems. When the need for globalization came around in the 19th century, it was a mess with competing systems and a large base of existing tools that would have to be scrapped in order to change. This is part of why the US is still not on the metric system, they didn't have their infrastructure wrecked in two world wars.

Unlike precision measurement, the need for precision time of day was introduced late into common use. While units of time existed (for example, the Chinese kè), they weren't needed by most people. For most of history, clocks were expensive and inaccurate, most people didn't have them and simply told the time by the Sun. If you needed to be more precise, ring a bell when it's time.

Then two things happened. First, the British Navy kept getting lost. You can figure out your latitude from the Sun, but to figure out longitude you need a very accurate clock. In the mid-18th century, John Harrison introduced a series of accurate clocks for navigation. Later trains became a thing. With trains came a need for accurate train schedules, and that meant accurate time keeping. So every train station needed a clock. The clock spread with British ships and the train.

The need for a universal system of timekeeping came pretty late. It happened after global trade and communications were a thing. The need for it was spread by Western technological inventions which would all be using the system we know today. And, unlike other measurements, there wasn't a large install base of existing time keeping devices.

As to why time didn't go metric along with everything else, they tried, but base 12 time was goo enough. Unlike the dizzying array of nonsense numbers in the Imperial system of measurement (12 inches to a foot; 3 feet to a yard; 1760 yards to a mile... or is it 8 furlongs to a mile... and those are the easy ones), time was already divided into fairly regular base 12 (some would argue superior to base 10). Even France quickly dumped decimal time.

Most likely because it wasn't until the modern era that anyone really needed, or could achieve, much precision with time measurements.

Prior to modern timekeeping, people pretty much lived on daily and seasonal schedule. The main thing you'd need to know was how close between sunup and sundown you were (and which side, but that's typically obvious by looking at the sky). So the world over, it makes sense to base your time off of a day (and a days off a year), and slices thereof.

That being said, time isn't really as "standard" as you imply. Originally the second was just 1/86400 of the time it took the earth to rotate back around to where the sun was roughly in the same position. After a while timkeeping instruments got accurate enough that the seasonal variances were a problem, so it got redefined based on the average "day" . After the development of Atomic (Cesium) clocks in the 1950's, timekeeping got so much better that the variances in the average day got to be an issue too. So now the second is defined based on the period of radiation coming of a cesium atom (the clocks are right, not the Earth!), so periodically extra fudge ("leap") seconds will be added to a day to keep it from drifting from what the rotation of the earth has actually been doing.

And then of course when the railroads came in the 19th century, the differences in time of day at the various stops became a problem. So the railroads started placing their stops in "time zones" for timetable purposes, rather than having to worry about using hundreds of different time offsets along all their routes. Generally these zones are in whole multiples of hours offset from the GMT zone, but not always.

Even with this, you often have to consult a map (or the locals) to know which zone you are in. But wait, we're not done! There's Daylight Savings Time. Not every area of a time zone will use it, and when they do then when they go on or off of it can vary from place to place (and year to year).

As for days, months, and years, until quite recently there were multiple different calendar systems in worldwide use. The Soviets switched Russia off of theirs onto the Gregorian one in 1918, although for a while they got rid of Sundays to try to retard theism. The Chinese also officially switched to Gregorian, but also continue to use their own native calendar for many purposes.