system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How did time become standardized across the world?

How did we get all of the countries to accept the hour/minute/second system?
gpt
Multiple cultures simultaneously developed a system of 'hours' equal to or divisible by 12.

- The ancient Egyptians divided the day into 12 equal parts, 10 during the day and 2 at twilight. 
- An ancient Greek astronomer, Andronicus of Cyrrhus, built a structure called the Tower of Winds which encompassed sundails, water clocks, and other timekeeping mechanisms to divide the day into 24 equal parts.
- In ancient China, the day was divided into 12 equal parts.
- In ancient India, the day was divided into 24 equal parts, 12 for 'day' and 12 for 'night'

Why 12 in so many different places? That's the number of lunar cycles in a year, which is where the concept of 12 months comes from. This made it easy to standardize 24 hours for a day around the world, as virtually everyone was already using 24 hours, or could easily adapt to it by dividing their 12-hour days by 2. Using a standard time system made trade and commerce much easier. This 'basis of 12' system is known as the 'duodecimal system', and comes from prehistoric lunar calenders.

That explains hours, but how did we get 60 minutes in an hour?

The ancient Greek astronomers relied on methods by their predecessors in Mesopotamia, who based all of their calculations on a sexagesimal system, or based off of the number 60, which can be traced back to Sumeria in about 2000 BC. That heritage is also why we have 360 degrees in a circle - as a result of the base 60 system.

Minutes and seconds came along a lot later than hours did. Before this, hours would be divided up into halves, thirds, quarters, what have you depending on need and region. There was no need for this level of precision for much of history.

Minutes and seconds were introduced sometime around the advent of mechanical clocks in the 14th century. There was a need for more accurate sub-units of hours. The world was still using the base 60 system at the time, and so hours were divided into 60 equal parts called minutes, which themselves were divided into 60 seconds. The base 60 system is also why clock faces are round.

Humanity transitioned from the base 60 system to a decimal system, or base 10, around the 17th century. It is easier to divide things by 10 than it is by 60. The sub-unit of seconds was not needed before this transition. That's why a second is divided into 1,000 milliseconds, which can be divided by 10, not 60 or 12.

Hope that was ELI5 enough!