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The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin ante meridiem, translating to "before midday") and p.m. (from Latin post meridiem, translating to "after midday"). [1] [2] Each period consists of 12 hours numbered: 12 (acting as 0), [3] 1, 2, 3, 4
Four studies carried out in 1997 found that subjects who were given tests on differentiating traffic signs had longer reaction times when tested at 3:00 pm and 6:00 pm than at 9:00 am and 12:00 pm. These trends are held across all four studies and for both complex and abstract questions. [ 18 ]
12 (noon) is normally 12 PM for the whole afternoon hour between 12:00 and 12:59. 13..23 (afternoon and start of night) is unambiguously 01..11 PM. 24 (midnight at end of day) is normally 12 AM (but the date must be adjusted when using the 12-hour format).
In Czech quarters and halves always refer to the following hour, e.g. čtvrt na osm (quarter on eight) meaning 7:15, půl osmé (half of eight) meaning 7:30 and tři čtvrtě na osm (three-quarters on eight) meaning 7:45. This corresponds to the time between 7:00 and 8:00 being the eighth hour of the day (the first hour starting at midnight).
In these cases, exact and unambiguous communication of time is critical. If someone mistakes 5:00 AM for 5:00 PM in a hospital for example, when medication or other medical treatment is needed at a certain time, the outcome could be dire. Thus 24-hour time (5:00 PM written as 17:00) is used.
The time-of-day abbreviations (which are generally lowercase only) are handled in various conflicting styles, including "a.m." and "p.m." with a space between the time and the abbreviation ("1.45 p.m."); [8] "am" and "pm" with a space ("1.45 pm" – recognised as an alternative usage by Oxford); [8] and the same without a space ("1.45pm ...
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The first mechanical public clocks introduced in Italy were mechanical 24-hour clocks which counted the 24 hours of the day from one-half hour after sunset to the evening of the following day. The 24th hour was the last hour of day time. [11] From the 14th to the 17th century, two systems of time measurement competed in Europe: [12] [13 ...