Date and time notation in the United States

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") and 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, twenty 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 globalization. The Chicago Manual of Style discourages writers from writing all-numeric dates in this format, since it is not comprehensible to readers outside the United States.[5]

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 1942" (sometimes with hyphens).[5] 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. "19 07 42"). Visas and passports issued by the U.S. State Department also use the day-month-year order for human-readable dates and year-month-day for all-numeric encoding, in compliance with the International Civil Aviation Organization's standards for machine-readable travel documents.[6][7]

The fully written "day-month-year" (e.g., 25 August 2006) in written American English is becoming more common outside of the media industry and legal documents, particularly in university publications and in some internationally influenced publications as a means of dealing with ambiguity. The Chicago Manual of Style recommends it for material that requires many full dates, since it does not require commas.[8] Most Americans still write "August 25, 2006" in informal documents. 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 U.S.'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[9]), 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 for correspondence with civilians.[10] 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. Many holidays and observances are identified relative to the day of the week on which they are 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". Some such definitions are more complex. For example, Election Day is defined as "the Tuesday next after the first Monday in the month of November" or "the first Tuesday after November 1".[11] Calendars mostly show Sunday as the first day of the week.

While most countries use the 24-hour clock, the United States differs in that it uses the 12-hour clock 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. (Business events, which are increasingly scheduled using groupware calendar applications, are less vulnerable to 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. It is also not uncommon for AM and PM to be shortened to A and P.

• Date format by country
• ISO 8601