Military Time: Para que serve e quando usar?

Minha ex-namorada americana os escrevia sem os pontos: 2130, 0820, 1750, 2300 etc

>> O trecho a seguir, retirado da Wikipedia, contém boas explicações:

In Canada and the United States, the term "military time" is a synonym for the 24-hour clock. In these regions, the time of day is customarily given almost exclusively using the 12-hour clock notation, which counts the hours of the day as 12, 1, ..., 11 with suffixes "a.m." and "p.m." distinguishing the two diurnal repetitions of this sequence. The 24-hour clock is commonly used there only in some specialist areas (military, aviation, navigation, tourism, meteorology, astronomy, computing, logistics, emergency services, hospitals), where the ambiguities of the 12-hour notation are deemed too inconvenient, cumbersome, or outright dangerous, with the military's use being the most famous example. The term "military time" has no particular meaning in most other regions of the world, where the 24-hour clock has long become a common element of everyday civilian life.

I understand that railroad workers in the USA use the same system. The employees, not the schedules. It reduces the possibility of misunderstandings that could lead to accidents and deaths. 10:45 AM is always "ten forty-five" and never "fifteen minutes before eleven" in "railroad time". Obviously this is an effort to standardize how time is told, and make it easily understood.

In the military you will also hear "oh-dark-hundred" or "oh-dark-thirty hours". It's a joke. It means "some time before dawn".

The US Navy apparently stills uses "bells" onboard ships. A "bell" is equal to thirty minutes. A watch (period of duty, work) lasts "eight bells". In the old days, a bell would be rung every thirty minutes: once after thirty minutes, twice after and hour, and so forth.

http://www.navy.mil/navydata/questions/bells.html

Time as marked by the bells The use of the bells to mark the time stems from the period when seamen (1) could not afford a personal time piece (i.e. - a watch) and (2) even if they could, they had no idea on how to tell time with such an instrument. The bells mark the hours of the watch in half-hour increments. The seamen would know if it were morning, noon, or night. Each watch* is four hours long and the bells are struck thus:

Mid Morning Forenoon Afternoon Dogs* First
0030 - 1 bell 0430 - 1 bell 0830 - 1 bell 1230 - 1 bell 1630 - 1 bell 2030 - 1 bell
0100 - 2 bells 0500 - 2 bells 0900 - 2 bells 1300 - 2 bells 1700 - 2 bells 2100 - 2 bells
0130 - 3 bells 0530 - 3 bells 0930 - 3 bells 1330 - 3 bells 1730 - 3 bells 2130 - 3 bells
0200 - 4 bells 0600 - 4 bells 1000 - 4 bells 1400 - 4 bells 1800 - 4 bells 2200 - 4 bells
0230 - 5 bells 0630 - 5 bells 1030 - 5 bells 1430 - 5 bells 1830 - 5 bells 2230 - 5 bells
0300 - 6 bells 0700 - 6 bells 1100 - 6 bells 1500 - 6 bells 1900 - 6 bells 2300 - 6 bells
0330 - 7 bells 0730 - 7 bells 1130 - 7 bells 1530 - 7 bells 1930 - 7 bells 2330 - 7 bells
0400 - 8 bells** 0800 - 8 bells 1200 - 8 bells 1600 - 8 bells 2000 - 8 bells 2400 - 8 bells

Notes: * - The period from 1600 to 2000 is split into two dog watches. These watches run from 1600 to 1800 and from 1800 to 2000. This alternates the daily watch routine so Sailors on the mid-watch would not have it the second night, and, the split also gives each watchstander the opportunity to eat the evening meal.
** - The end of the watch is considered at 8 bells, hence the saying "Eight Bells and All Is Well."

Acredito que quando falamos da Força Aerea, quando o piloto fala, O avião está a 12h por exemplo, ele quer dizer que o avião está na posição acima do avião, assim imaginamos um relogio atrás do avião e sempre que ele fala horas, significa na verdade a posição que outra aeronave está em relação a dele.

A military friend of mine describes how time is spent in the service:

"Hurry up and wait."

Thanks guys for the answer ...

Actually "12 o'clock" means "straight ahead". If you are calling attention to something straight ahead and below, it's "12 o'clock low." If it is straight ahead and above the airplane, then it's "12 o'clock high", which is also the title of an excellent book and movie about the US Army Air Corps in Europe during World War II.

Note that these terms refer to locations, not to times.

A common farewell among fighter pilots is "Watch your six." Any ideas what that means? It is short for "Watch for enemy planes behind you." "Six o'clock" is directly behind an airplane.