Date and time notation in the United States
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In the United States, dates are traditionally written in the "month day, year" order, that is, in neither descending nor ascending order of significance. (In computing, this would be called a "middle-endian" order.) This order is used in both the traditional all-numeric date (e.g., "12/31/99" or "12/31/1999") (said with all cardinal numbers) as well as in the expanded form (e.g., "December 31, 1999") (usually spoken with the year as a cardinal number and the day as an ordinal number; e.g., "December thirty-first, nineteen ninety-nine"), with the historical rationale that it is indeed big-endian with respect to the month and day, as the year was often of lesser importance. The most commonly used separator in the all-numeric form is the slash (/), although the hyphen (-) is also common. Periods (.) have also emerged in the all-numeric format recently due to globalization.
The day-month-order has increased in usage notably since the early 1980s. The month is usually written as a name, as in "12-Dec-1999". 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. Visas and passports issued by the U.S. State Department also use this format.
The ISO 8601 yyyy-mm-dd format is also used within the Federal Aviation Administration and military because of the need to eliminate ambiguity. The fully written "day month year" (e.g., 12 March 2005) in written American English is starting to become 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 write "March 12, 2005". Speaking the "day month year" format is still rarely used, with the exception of the Fourth of July.
The ISO 8601 date notation YYYY-MM-DD is popular in some computer applications because it greatly reduces the amount of code needed to resolve and compute dates. It may be considered less of a break with tradition by U.S. users, since it preserves the familiar month-day order. Two U.S. standards mandate the use of ISO 8601-like formats: ANSI INCITS 30-1997 (R2008); and NIST FIPS PUB 4-2 (FIPS PUB 4-2 withdrawn in United States 2008-09-02), the earliest of which is traceable back to 1968.
Weeks are generally referred to by the date of some day within that week (e.g., "the week of March 5"), rather than by a week number. 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 with some computer software. The suffix "a.m." or "p.m." (often represented as AM and PM) is appended universally in written language. Where this is inconvenient typographically (e.g., in dense tables), different fonts or colors are sometimes used instead of a.m./p.m. 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. Due to ambiguity of the 12-hour notation at noon and especially midnight, events are sometimes scheduled at "11:59 p.m." or "12:01 a.m." instead of 12:00 a.m. to remove ambiguity. 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, avoid such ambiguity, since the software itself takes care of the naming conventions.) 
The 24-hour notation is rarely used so far in the U.S. in daily life.
It is best known there 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 is sometimes 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, 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.
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".
- [dead link] . National Institute of Standards and Technology.
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