When did the U.S. Army start saying "oh-six-hundred" for "6 AM"?

Upvote:8

My grandfathers were both in wars. One in the Korean War, one in WW2. I was in Iraq and Afghanistan. None of us have ever said Oh-x-hour. It's always been zero. Like zero-6 hundred. It was used in WW1 from what I know, but wasn't made official until the 1940s. There's a movie called Zero Dark Thirty, not Oh Dark Thirty because no one says Oh Dark.

Upvote:42

The US military did not use the 24-hour clock system at the time depicted in that movie (emphasis mine).

During World War I, the British Royal Navy adopted the 24-hour clock in 1915, and the Allied armed forces followed soon after,[22] with the British Army switching officially in 1918.[24] The Canadian armed forces first started to use the 24-hour clock in late 1917.[25] In 1920, the United States Navy was the first United States organization to adopt the system; the United States Army, however, did not officially adopt the 24-hour clock until World War II, on July 1, 1942.[26]

I haven't located the original order yet, but did find mention of a training circular put out within a month, from a US Army publication from 1942. See circular #51, first on the list:

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Another US Government document, labelled Information Digest, published on 17 June 1942 posts notice of when the US Army will start using the 24-hr clock (center paragraph): enter image description here

This confirms July 1st as starting date for the official use of the 'military time' system, for all

Official messages, dispatches, orders and reports...

How early did others adopt this system?

The above linked Wikipedia article mentions that the 24-hour clock was adopted in organizations within other nations as early as the late 19th century (Italy in 1893). None are listed as early as the 1830s:

A report by a government committee in the United Kingdom noted Italy as the first country among those mentioned to adopt 24-hour time nationally, in 1893.[22] Other European countries followed: France adopted it in 1912 (the French army in 1909), followed by Denmark (1916), and Greece (1917). By 1920, Spain, Portugal, Belgium, and Switzerland had switched, followed by Turkey (1925), and Germany (1927). By the early 1920s, many countries in Latin America had also adopted the 24-hour clock.[23] Some of the railways in India had switched before the outbreak of the war.[22]

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