Comparison of the 12-hour and 24-hour clocks

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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.

[edit] Advantages of the 12-hour clock

A typical analog 12-hour clock
A typical analog 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. More rarely used 24-hour analog dials necessarily have smaller spaces between hours, which can be harder to read, particularly on a small dial.
  • Restricting the hour numbers to the range from 1 to 12 can accommodate a preference for monosyllabic words, which applies to most originally European languages (including English, French, German, Danish, Swedish, Norwegian, Spanish, Portuguese...), in which most of the 12 are monosyllabic, opposed to the 13–24 range, in which fewer are monosyllabic.
  • The use of 12 rather than 0 for the first hour of each period avoids using a leading zero.
  • The practice, used with the 24-hour system, to indicate midnight at the end of a date by 24:00, in addition to the notation 00:00 for the start of the next day, creates two different notations for the same instant. The 12-hour system as used in practice has one and only one notation for each instant.[citation needed]
  • Clocks with a chime universally chime between hours 1 to 12, but deviate in approach for hour 0 and hours 13 to 23. While it is not possible to chime zero times, clocks may in fact chime at the end of 23:59 and again at 1:00.[citation needed]
  • Sunrise and sunset are, roughly, centered around 6:00 in their respective halves of the day during the course of a year. That is, at an equinox, and disregarding civil time (including DST) in favor of apparent solar time, the sun will rise at 6:00 a.m. and set at 6:00 p.m. At the local summer solstice, the sun might rise at, say, 4:00 a.m. and set at 8:00 p.m., and vice-versa (8:00 a.m. to 4:00 p.m.) at the local winter solstice. There are many reasons why this is not exact (atmospheric refraction, the fact the sun is a disc and not a point, time zones, daylight saving time, and so forth), but it still can be a useful approximation under certain conditions.
  • The use of a leading zero, as is often[citation needed] used for 00:00 to 09:59, is awkward to pronounce out loud. That pronunciation is common mostly in the US military, not in countries primarily using the 24-hour clock.

[edit] Advantages of the 24-hour clock

An analog 24-hour watch
An analog 24-hour watch
A digital clock showing minutes 00:01 to 24:00 (rather than the standard 00:00 to 23:59)
A digital clock showing minutes 00:01 to 24:00 (rather than the standard 00:00 to 23:59)
  • It is possible to differ between night and day even in sealed environments, whereas the 12-hour system (at least on clocks with dials) relies on sight.
  • There is no ambiguity between noon or midnight, 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. till 3:30 p.m. is 5 hours. From 10:30 till 15:30 indicates this more clearly.
  • The 24-hour notation does not require the suffix "a.m." or "p.m.", making it shorter, which saves space in tables.
  • 24-hour notation in string form is easier for computers to sort, and it's also less time expensive to convert the time from string form to Unix time.
  • 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. transition.
  • 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.
  • 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.".