[1] In traditional American usage, dates are written in the month–day–year order (e.g. February 16, 2025) with a comma before and after the year if it is not at the end of a sentence[2] and time in 12-hour notation (12:04 am).
[5] These forms are increasingly common in American professional, academic, technological, military, and other internationally oriented environments.
This order is used in both the traditional all-numeric date (e.g., "1/21/24" or "01/21/2024") and the expanded form (e.g., "January 21, 2024"—usually spoken with the year as a cardinal number and the day as an ordinal number, e.g., "January twenty-first, twenty twenty-four"), with the historical rationale that the year was often of lesser importance.
The Chicago Manual of Style discourages writers from writing all-numeric dates, other than the year-month-day format advocated by ISO 8601, as it is not comprehensible to readers outside the United States.
[5] Many genealogical databases and the Modern Language Association citation style use this format.
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.
It is also commonly used in software cases where there are many separately dated items, such as documents or media, because sorting alphanumerically will automatically result in the content being listed chronologically.
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.)
The United States 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.
(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.)
It is also not uncommon for AM and PM to be shortened to A and P. The 24-hour clock is used in military, public safety, and scientific contexts in the United States.
[15] The 24-hour notation is also widely used by astronomers, hospitals, public safety personnel (police, fire department, EMS), 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.