Comparison of the 12-hour and 24-hour clocks

Different stylistic conventions and habits exist around the world in order to express time in written and spoken language. The two main systems are the 24-hour clock or the 12-hour clock notation. When used correctly there is no ambiguity between times in the morning or evening in either system; however, ambiguity still exists when the clock being used is unstated and, with the 12-hour notation, for noon and midnight(s).
Conventions for expressing time can also differ substantially for written communication and spoken language.
ISO 8601 defines an unambiguous written format for time based on the 24-hour notation. This notation has been adopted by many countries as a national standard (e.g., BS EN 28601 in Britain and other EU countries, ANSI INCITS 30-1997 (R2003) and FIPS PUB 4-2 in the United States) and is, in particular, widely used in computer applications.
Advantages of the 12-hour clock
* Typical analog clock faces are divided into 12 hours, with each hour point in addition serving as five minutes, which lines up well with a 12-hour system.
* Restricting the hour numbers to the range from 1 to 12 can accommodate a preference for monosyllabic words, which applies to many European languages (including English, French, German, Dutch, Danish, Swedish, Norwegian), in which most of the 12 are monosyllabic, opposed to the 13-24 range, in which none of those numbers are monosyllabic.
* The use of 12 rather than 0 for the first hour of each period avoids using a leading zero.
* Clocks with a chime universally chime between hours 1 to 12, but deviate in approach for hour 0 and hours 13 to 23.
* Sunrise and sunset are, roughly, centered around 6:00 in their respective halves of the day during the course of a year.
Advantages of the 24-hour clock
* There is no ambiguity between noon or midnight, whereas confusion exists on this issue with the 12-hour system.
* There is no ambiguity between midnight at the start and end of a date, whereas confusion exists on this issue with the 12-hour system.
* The duration of time intervals is easier to see in the 24-hour notation. From 10:30 a.m. until 3:30 p.m. is 5 hours. From 10:30 until 15:30 indicates this more clearly.
* The transition from 23:59 to 00:00, provides a clear reminder that a new date starts, which is less obvious in the equivalent 11:59 p.m. to "12:00 a.m." (midnight) transition.
* The 24-hour notation does not require the suffix "a.m." or "p.m.", making it shorter, which saves space.
* Sorting is more obvious in 24-hour notation. The lexicographical order is the same as the chronological order, a property exploited by ISO 8601. The sequence of 24-hour times 00:30 < 11:30 < 12:30 < 23:30 stays in the same sequence by a lexicographical sort. The same sequence expressed in 12-hour system, would be ordered differently by a lexicographical sort: "11:30 a.m." < "11:30 p.m." < "12:30 a.m." < "12:30 p.m.".
* As digital clocks sometimes provide little indication (often only a dot with no visible label in the dark) of whether the displayed time is a.m. or p.m., there is a tendency to incorrectly set the alarm or the time (or both) on them. The 24-hour system does not have this issue.
*It is possible to differentiate between night and day even in sealed environments, and near the poles with midnight sun, whereas the 12-hour system (at least on clocks with dials) relies on sight, and near sunrise/sunset a guess if the sun going up or down.

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