Time Zones were established to help deal with the fact that day and night occurs at different times on Earth due to which function?

Interestingly, many French maps showed zero degrees in Paris for many years despite the International Meridian Conference’s outcomes in 1884. GMT was the universal reference standard – all other times being stated as so many hours ahead or behind it – but the French continued to treat Paris as the prime meridian until 1911. Even so, the French defined their civil time as Paris Mean Time minus 9 minutes and 21 seconds. In other words, this was the same time as GMT.

In 1972, Coordinated Universal Time (UTC) replaced GMT as the world's time standard. France did not formally use UTC as a reference to its standard time zone (UTC+1) until August in 1978.

Standard time, in terms of time zones, was not established in United States law until the Act of March 19, 1918. The act also established daylight saving time in the nation. Daylight saving time was repealed in 1919, but standard time in time zones remained in law, with the Interstate Commerce Commission (ICC) having the authority over time zone boundaries.

Many countries started using hourly time zones by the late 1920s. Many nations today use standard time zones, but some places use 30 or 45 minute deviations from standard time. Some countries such as China use a single time zone even though their territory extends beyond the 15 degrees of longitude.