Now that we've covered a part of origin of calendars, let's move on to how time was originated.
Unstoppable Time
How did we know that a day consists of 24 hours, and of course, this is related to the previous post; hence, different countries and tribes might use different methods to calculate hours. Like the Sumer calendar, it is (probably) agreed that a minute is 60 seconds and each hour is 60 minutes, but why 60? Why not 50 or 20?
EXTRA NOTES: 60 is a widely used number, such as; angles, time, and geographic coordination. The number 60, a superior highly composite number, has twelve factors, namely {1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60}, of which 2, 3, and 5 are prime numbers. With so many factors, many fractions involving sexagesimal numbers are simplified. For example, one hour can be divided evenly into sections of 30 minutes, 20 minutes, 15 minutes, 12 minutes, 10 minutes, 6 minutes, 5 minutes, 4 minutes, 3 minutes, 2 minutes, and 1 minute. 60 is the smallest number that is divisible by every number from 1 to 6; that is, it is the lowest common multiple of 1, 2, 3, 4, 5, and 6.
One hour of time is divided into 60 minutes, and one minute is divided into 60 seconds. Thus, a measurement of time such as "3:23:17" (three hours, 23 minutes, and 17 seconds) can be interpreted as a sexagesimal number, meaning 3×602 + 23×601 + 17×600. As with the ancient Babylonian sexagesimal system, however, each of the three sexagesimal digits in this number (3, 23, and 17) is written using the decimal system.
Similarly, the practical unit of angular measure is the degree, of which there are 360 in a circle. There are 60 minutes of arc in a degree, and 60 arcseconds in a minute; hence the origin concept of clocks and stopwatch (the analogue stopwatch)
(Source: Wikipedia)
The 24 hours concept was actually counted by astronomers in the old days, and there were various methods in counting; counting from dawn, sunset, noon, and midnight. And again, each country have different methods in counting it.
For those which counted from dawn, sunrise marked the beginning of the first hour (the zero hour), the middle of the day was at the end of the sixth hour and sunset at the end of the twelfth hour. This meant that the duration of hours varied with the season. In the Northern hemisphere, particularly in the more northerly latitudes, summer daytime hours were longer than winter daytime hours, each being one twelfth of the time between sunrise and sunset. These variable-length hours were variously known as temporal, unequal, or seasonal hours and were in use until the appearance of the mechanical clock, which furthered the adoption of equal length hours. Not to mention, this concept might've lead some astronomers to think that the Earth's revolution orbit was NOT a circle shaped one, which was then proven and now lives the concept of elliptical shape orbit. And also, this method is used in Jewish law.
Sundial
Meanwhile, unlike the previous one, the Babylonians count hours using sundials, and divide the day and night into equal 24 hours.
For many centuries, up to 1925, astronomers counted the hours and days from noon, because it was the easiest solar event to measure accurately. An advantage of this method (used in the Julian Date system, in which a new Julian Day begins at noon) is that the date doesn't change during a single night's observing.
In the modern 12-hour clock, counting the hours starts at midnight and restarts at noon. Hours are numbered 12, 1, 2, ..., 11. Solar noon is always close to 12 noon, differing according to the equation of time by as much as fifteen minutes either way. At the equinoxes sunrise is around 6 A.M., and sunset around 6 P.M.
In the modern 24-hour clock, counting the hours starts at midnight and hours are numbered from 0 to 23. Solar noon is always close to 12:00, again differing according to the equation of time. At the equinoxes sunrise is around 06:00 and sunset around 18:00.
While astronomers are busy counting how many hours exist in a day, other scientists uses the concept of time as a unit to count other stuffs, like velocity, acceleration, force, manpower, etc. And so, it was probably universal for the world after they see their results and compare to one another, and agreed that one day consists of 24 hours.