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Time-keeping was invented by humans. In the past, timekeeping was as simple as checking the position of the sky. Then came sundials and finally, clocks.

It was the advent of the railroads that necessitated developing and adopting a method of time-keeping that was consistent not only within countries and continents, but among all countries and continents, or in other words, all over the world. When the aviation industry emerged in the early 20th century, it became apparent to those inside and outside of the industry that a way to standardize time-keeping world-wide, skewed to the aviation industry, was desperately needed.

The adoption of Greenwich Mean Time as the starting point for worldwide timekeeping worked well as long as humans crossed time zones on the ground. After all, one can only travel so fast on a train or in an automobile. But when planes began flying across time zone after time zone within the course of one flight, aviators needed a way to determine quickly what time it was without having to take into consideration whether it was day or night.

The solution came in using a 24-hour clock, with zero o’clock defined as midnight Greenwich Mean Time (GMT). Then you simply added the number of time zones east of GMT of your current position or subtracted the number of time zones west of GMT of your current position to arrive at the aviation industry standard time, known as Zulu Time.

Why is this important? Because pilots could easily synchronize arrival and departure times and quickly heed warnings of adverse weather systems. Just like pilots world-wide must communicate with air traffic control in English—a common language necessary for safety and air traffic control—using Zulu time enables pilots to quickly determine what time events in other time zones will affect them. So if a pilot flying over the center of North America is told that a weather system will arrive at approximately 1900 hours Zulu Time, he or she can quickly determine that the weather front will arrive at 1300 hours local time.

How do you translate Zulu Time to local time? Each time zone is given a letter from the military phonetic alphabet, moving east from Greenwich, England. The first time zone to the east of Greenwich is “Alpha.” Continuing east around the globe, each subsequent time zone follows alphabetically.

Over central North America, the Central time zone is named Sierra in military/aviation time. Since Sierra/US Central time is six time zones west of Greenwich Mean/Zulu time, the pilot simply consults a chart, which says to subtract six hours from the given Zulu time to arrive at the corresponding time Sierra/US Central Time. And since a 24-hour clock is used, the pilot doesn’t have to waste time figuring out if the time in question is A.M. or P.M.