Date and time notation in the United States
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Date and time notation in the United States differs from that used in other English-speaking countries. Traditionally, dates are written in the "month day, year" order ("December 12, 1999"), and time in 12-hour notation ("11:00 p.m.") The internationally more common notations are used in some professional environments, such as in the military.
In the United States, dates are traditionally written in the "month, day, year" order, with neither increasing nor decreasing order of significance. This order is used in both the traditional all-numeric date (e.g., "1/21/16" or "01/21/2016") as well as in the expanded form (e.g., "January 21, 2016") (usually spoken with the year as a cardinal number and the day as an ordinal number; e.g., "January twenty-first, two thousand sixteen"), with the historical rationale that the year was often of lesser importance. The most commonly used separator in the all-numeric form is the slash (/), although the hyphen (-) and period (.) have also emerged in the all-numeric format recently due to globalisation.
The day-month-year order has been increasing in usage since the early 1980s. The month is usually written as an abbreviated name, as in "19-Jul-1922". Many genealogical databases and the Modern Language Association citation style use this format. When filling in the Form I-94 cards and new customs declaration cards used for people entering the U.S., passengers are requested to write pertinent dates in the numeric "dd mm yy" format (e.g. "21 01 16"). Visas and passports issued by the U.S. State Department also use this format. In the food industry, many companies in the US are starting to print expiration dates in the "dd mmm yyyy" format (e.g. "19 NOV 2016").
The fully written "day-month-year" (e.g., 25 May 2006) in written American English is becoming more common outside of the media industry and legal documents, particularly in university publications and in some international-influenced publications as a means of dealing with ambiguity. However, most Americans (in most informal documents) would still write "May 25, 2006". Speaking the "day month year" format is still somewhat rare, with the exception of holidays such as the Fourth of July.
The year-month-day order, such as the ISO 8601 "YYYY-MM-DD" notation is popular in computer applications because it reduces the amount of code needed to resolve and compute dates. It is also commonly used in software cases where there are many separately dated items, such as documents or media, because sorting alphabetically will automatically result in the content being listed chronologically. Switching the US's traditional date format from month-day-year to year-month-day may be considered less of a break, since it preserves the familiar month-day order.
Two U.S. standards mandate the use of year-month-day formats: ANSI INCITS 30-1997 (R2008); and NIST FIPS PUB 4-2 (FIPS PUB 4-2 withdrawn in United States 2008-09-02), the earliest of which is traceable back to 1968. This order is also used within the Federal Aviation Administration and military because of the need to eliminate ambiguity.
The United States military normally uses the "dd mmm yyyy" format for correspondence. The common month-day-year format is used when corresponding with civilians. The military date notation is similar to the date notation in British English but is read cardinally (e.g. "Nineteen July") rather than ordinally (e.g. "The nineteenth of July").
Weeks are generally referred to by the date of some day within that week (e.g., "the week of May 25"), rather than by a week number. Holidays are an exception; such days are typically identified by a week number, relative to the day of the week on which the holiday is fixed, either from the beginning of the month (first, second, etc.) or end (last, and far more rarely penultimate and antepenultimate). For example, Thanksgiving is defined as being on "the fourth Thursday in November." Calendars mostly show Sunday as the first day of the week.
The U.S. differs from other countries in that it uses 12-hour notation almost exclusively, not only in spoken language, but also in writing, even on timetables, for airline tickets, and computer software. The suffixes "a.m." and "p.m." (often represented as AM and PM) are appended universally in written language. Alternatively, people might specify "noon" or "midnight", after or instead of 12:00 (12:00M for 00:00 and 12:00N for 12:00). (Business events, which are increasingly scheduled using groupware calendar applications, either use 24-hour time or avoid such ambiguity, since the software itself can be modified to take care of the naming conventions.)  Where the a.m/p.m. convention is inconvenient typographically (e.g., in dense tables), different fonts or colors are sometimes used instead. The most common usage in transport timetables for air, rail, bus, etc. is to use lightface for a.m. times and boldface for p.m. times.
The 24-hour notation is rarely used so far in the U.S. in daily life. If 24-hour time is used by an American citizen, he/she would simply refer the time as "military time".
It is best known for its use by the military, and therefore commonly called "military time". In U.S. military use, 24-hour time is traditionally written without a colon (1800 instead of 18:00) and in spoken language in the Army, but not the Navy, is followed by the word "hours" (e.g., "eighteen hundred hours").
The 24-hour notation is also widely used by astronomers and other communities such as public safety (police, fire, rescue), who would pronounce 18:35 "Eighteen Thirty Five", for example in two-way communications.
It is also used in hospitals, various forms of transportation, and at radio and other broadcast media outlets behind the scenes where scheduling programming needs to be exact, without mistaking AM and PM. In these cases, exact and unambiguous communication of time is critical. If someone mistakes 5:00 AM with 5:00 PM in a hospital for example, when medication or other medical treatment is needed at a certain time, the outcome could be critical. Thus 24-hour time (5:00 pm, 17:00) is used.
Some style guides and most people suggest not to use a leading zero with a single-digit hour; for example, "3:52 p.m." is preferred over "03:52 p.m.". (The leading zero is more commonly used with the 24-hour notation; especially in computer applications because it can help to maintain column alignment in tables and correct sorting order, and also because it helps to highlight the 24-hour character of the given time.)
Times of day ending in :00 minutes may be pronounced as the numbered hour followed by o'clock (e.g., 10:00 ten o'clock, 2:00 two o'clock, 4:00 four o'clock, etc.). This may be followed by the a.m. or p.m. designator, or might not be, if obvious. O'clock itself may be omitted, leaving a time like four a.m. or four p.m. Instead of "a.m." and "p.m.," times can also be described as "in the morning", "in the afternoon," "in the evening," or "at night."
The minutes (other than :00) may be pronounced in a variety of ways:
Minutes :01 through :09 are usually pronounced as oh one through oh nine. :10 through :59 are their usual number-words. For example, "9:45 a.m." is usually pronounced "nine forty-five" or sometimes "nine forty-five a.m."
Times of day from :01 to :29 minutes past the hour are commonly pronounced with the words "after" or "past", for example 10:17 being "seventeen after ten" or "seventeen past ten". :15 minutes is very commonly called "quarter after" or "quarter past" and :30 minutes universally "half past", e.g., 4:30, "half past four". Times of day from :31 to :59 are, by contrast, given subtractively with the words "to", "of", "until", or "till": 12:55 would be pronounced as "five to one" or "five of one". :45 minutes is pronounced as "quarter to", "quarter of", "quarter until", or "quarter till".
For example, "9:45 a.m." is often pronounced "fifteen till ten" or "quarter to ten", or sometimes "quarter to ten in the morning" (but rarely "quarter to ten a.m.").
However, it is always acceptable to pronounce the time using number words and the aforementioned "oh" convention, for example, 12:55 "twelve fifty-five", 12:09 "twelve oh-nine", 12:30 "twelve thirty", and 12:15 "twelve fifteen".
The Department of State and the Department of Defense timestamp their reports and messages with text date-time groups formatted as
DDHHMMZ MMM YY, where DD represents 2-digit day of the month, HHMMZ is 24-hour "Zulu time" (in GMT/UTC), MMM is abbreviated month and YY are the last two digits of the year. For example,
091630Z JUL 11 represents 16:30 UTC on 9 July 2011.
- [dead link] . National Institute of Standards and Technology.
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