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The History of Military Time: From Ancient Egypt to the U.S. Army

How the 24-hour day began in ancient Egypt, how railways and navies adopted 24-hour time, and when the Royal Navy, British Army, U.S. Navy and U.S. Army switched to military time.

By Free Military Time Calculator Team ·

Timeline of the 24-hour clock from ancient Egypt to 1942

Military time grew out of the 24-hour day of ancient Egypt and was adopted by navies and armies in the early 1900s: the Royal Navy in 1915, the British Army in 1918, the U.S. Navy in 1920 and the U.S. Army in 1942. Each adopted it for the same reason: orders with AM and PM caused mistakes.

The 24-Hour Day

Ancient Egyptians divided the day and the night into 12 hours each, giving 24 hours, and the hours changed length with the seasons. The Greek astronomer Hipparchus proposed 24 hours of equal length, the basis of every modern clock.

Railways and the 24-Hour Clock

Railways needed timetables that could not be misread. Sandford Fleming, the Canadian engineer who promoted standard time zones, also championed 24-hour time, and the Canadian Pacific Railway adopted the 24-hour clock in 1886. Many European railways followed, and 24-hour timetables are standard across Europe today.

Year Organization
1915 Royal Navy
1918 British Army
1920 U.S. Navy
1942 U.S. Army

The U.S. Army switched during World War II, when units across many time zones needed orders that every reader understood the same way.

Zulu Time and Standard Time Zones

The same era standardized time zones. Military forces later lettered each zone and made Zulu (UTC) the shared reference, so a message written as 1430Z means one moment worldwide.

Military Time Today

All U.S. military branches use the same 4-digit format from 0000 to 2359. Hospitals, police and fire departments, emergency medical services, aviation, shipping and computer systems use it every day, and the international standard ISO 8601 writes times on the 24-hour clock.

Why the Format Has Lasted

3 features keep military time in use after more than a century:

  • One time, one meaning: 0700 and 1900 cannot be confused.
  • Clear over radio: “fourteen thirty hours” is hard to mishear.
  • Easy math: durations subtract without AM/PM conversion.

Learn the basics in what is military time, or convert a time with the military time converter.