Different conventions exist around the world for date and time representation, both written and spoken.
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Differences can exist in:
International standard ISO 8601 (Representation of dates and times) defines unambiguous written all-numeric big-endian formats for dates, such as 1999-12-31 for 31 December 1999, and time, such as 23:59:59 for 23 hours, 59 minutes, and 59 seconds (one second before midnight).
These standards notations have been adopted by many countries as a national standard, e.g., BS EN 28601 in the UK and similarly in other EU countries, ANSI INCITS 30-1997 (R2008), and FIPS PUB 4-2 in the United States (FIPS PUB 4-2 withdrawn in United States 2008-09-02).[1] They are, in particular, increasingly widely used in computer applications, since the most to least significant digit order provides a simple method to order and sort time readings.
In terms of dates, most countries use the "day month year" format. In terms of people the big-endian form is also very common, since that is used in East Asia, Iran and partially in India.
The 24-hour clock enjoys broad everyday usage in most countries outside North America, Australia and the Philippines, at least when time is written or displayed. In some regions, for example most German, French and Romanian speakers, use the 24-hour clock today even when speaking casually, while in other countries the 12-hour clock is used more often in spoken form.
In other English-speaking regions, particularly former colonies of the United Kingdom, the 12-hour clock and 24-hour are used interchangeably in formal communications.
Most people in "24-hour countries" are so used to both systems that they have no problem switching between the two, perceiving "three o'clock" and "15:00" simply as synonyms. When speaking, a person may often pronounce time in 12-hour notation, even when reading a 24-hour display. It is also common that a person uses the 24-hour clock in spoken language when referring to an exact point in time ("The train leaves at fourteen forty-five …"), while using some variant of the 12-hour notation to refer vaguely to a time ("… so I will be back tonight sometime after five.").