Definitions of time

Measurement of time is a social construct. The flow of time is a physical reality, but the units humans use to track it are entirely manmade.

Origins of the Units of Time

The units of time are cultural and historical inventions.

The Number 60: The ancient Sumerians and Babylonians used a sexagesimal (base-60) number system. This was chosen to measure (60 minutes/60 seconds) because 60 is divisible by many numbers, making it easy to calculate fractions of time.
The 24-Hour Day: The ancient Egyptians divided the daylight into 10 hours, added 2 hours for twilight, and 12 hours for the night. They were the first civilization to divide the day into parts using advanced sundials. When time was standardized, this was adopted as the base system. (Note that 60 is divisible by 10, 2, and 12, making time easy to calculate in this system.)
The 7-Day Week: This is a social and cultural convention that was loosely based on the lunar cycle, but with no strict basis in astronomy. A "week" is a human-designed rhythm for labor and rest that endured since the Roman calendar in 321 CE, embedded in human cultures, systems, and practices.

Measuring Time

Apparent Solar Time

For most of human history, time was defined by celestial movement.

The Day: Interval between two successive noons (when the sun is at its highest point)
The Month: Approximately 29.5 days; originally tracked by the phases of the moon
The Year: Marked by seasons and equinoxes and based on the Earth's orbit around the sun

Because the Earth's orbit is elliptical and its axis is tilted, solar days vary in length throughout the year. To solve this, humans created Mean Solar Time, which averages the length of all solar days to 24 hours.

Greenwich Mean Time (GMT)

Before standard time was established, each town set its clocks to high noon. However, noon in one city could be 10+ minutes behind another city merely 50 miles away. The invention of the railroad made this localized system obsolete. In 1884, the International Meridian Conference established the Royal Observatory in Greenwich, London, UK, as the Prime Meridian (0° longitude). Time zones were divided into 15-degree increments of longitude, each representing 1 hour.

We use Coordinated Universal Time (UTC) as the primary standard by which the world regulates clocks and time. It was confusing that GMT started at noon, but midnight almost everywhere else. UTC was introduced in 1928 to denote that the day started at midnight.

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