Time zones are not a natural phenomenon.

People did not become aware that there were different time zones. People created time zones as an artificial construction because it was convenient.

Time zones were first proposed in the US in 1863, to standardize the system of ‘railroad time’, (in which each railroad had set a standard time to use across its network, but all these standard times were different). This was not implemented until 1883. Time zones were standardized across the whole country in 1918 when the “Standard Time Act” was passed by Congress.

Of course, people were long aware that time varied with geographical position. (4 minutes of time for every degree of longitude), and that noon occurred at different times as one travelled east or west.

Before clocks, people used apparent or “true” solar time. Setting noon as when the sun was highest in the local sky (and the shadow on a sun dial was shortest).

After clocks (circa early 19th century), they tended to use “mean solar time”, as this gives days of the same length, whereas the length of days varies through the year with “true” solar time (due to variability in the earth’s orbit and rotation).

When railroads were developed, the use of different time at every different place along the route became troublesome and made scheduling difficult. In 1840, in Britain, the Great Western Railroad standardized on Greenwich Mean time all along its route, and by 1847 all British railroads had followed suit. By 1855, almost all of Britain was using standard time.

This however was not the invention of time zones (it might have been the invention of one time zone).

By 1929 almost all countries had declared one or more standard time zones referenced to GMT (Nepal was the last to do so, in 1986.

Thus the current system of time zones was constructed. People became aware of the concept of time zones in the 19th century, and built the system of time zones by the end of the 20th century.

To add a little to mormengil: by the time of Erastosthenes the Greeks had worked out the approximate diameter of the world, and any traveler who knew this, that the world was round, could also realize that sailing in one direction would make days ( defined as sunrise to sunset) shorter and , in the other, longer. And that there would always be a dawn someplace on the globe at the same time as a sunset.