AM,PM的來源

a.m.=ante meridiem(=before noon) 上午
p.m.=post meridiem 下午

am pm 或者 a.m. p.m. 都得，兩個都係拉丁文o既 abbreviation

a.m. (MORNING), (am) (ante meridiem: a.m., from Latin, literally "before midday") (adverb)
used when referring to a time between twelve o'clock at night and twelve o'clock in the middle of the day:
e.g. The first election results are expected around 1 a.m.

p.m. (TIME), pm (ABBREVIATION) (post meridiem: p.m., "after midday")
used when referring to a time in the afternoon or evening or at night:
e.g.1 We'll be arriving at about 4.30 p.m.
e.g.2 The 6pm train is usually very crowded.

上午，（Ante Meridiem）一個時間的單位用於十二小時制，拉丁文：Ante Meridiem 意為中午之前。

ante是字首，意為在時間、順序、位置上在前；post是字首，意為在時間、順序上在後。meridiem意為正午。所以ante meridiem是before noon; post meridiem是after noon.英文寫作antemeridian 及postmeridian.

在古代，像其他語言的縮略語一樣，英語縮略語多見於古幣、徽章、圖案和碑文，因為要在這些小小的物體上刻劃出較為複雜的內容，只有用縮略語。而這些古幣、 徽章、圖案等首選是因為商務交際的需要人們才使用的，可以說商務縮略語首選產生於商務這一語域中。縮略語也曾作為一種記事的符號和密碼。如古羅馬人把RSP( Roman Senate and People)當作“古羅馬的元老院”的縮寫語，把拉丁語Anno Domini (西元)縮寫為A.D.，把ante meridiem 和post meridiem 分別縮寫為a.m.和p.m.，這類古老的縮略語早已進入了詞典，廣泛被人們所使用。

下午，（post meridiem） 一個時間的單位用於十二小時制，拉丁文：Ante Meridiem 意為中午之後。

其實AM./PM.都不是英文，而是阿拉丁文。
AM=AUTE MERIDIEM=正午前
PM=POST MERIDIEM=正午後
MERIDIEM-->正午後
AUTE-->前
POST-->後
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明白未！KU...

2007-03-02 22:54:02 補充：
第一行不是阿拉丁文，應是拉丁文。不便之處，敬請原諒。>=箭嘴

我們是用拉丁文定義的
A.M., ante meridiem
P.M., post meridiem
更多關於12小時制計時:
The 12-hour clock is a timekeeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (a.m., from Latin, literally "before midday") and post meridiem (p.m., "after midday"). Each period consists of 12 hours numbered 12 (acting as a zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. The a.m. period runs from midnight to noon, while the p.m. period runs from noon to midnight. The most common convention is to assign 12 a.m. to midnight (at the beginning of the day) and 12 p.m. to noon, defining both half days to have a closed (inclusive) beginning and open (exclusive) end. 12 noon and 12 midnight can more clearly express these times (except that for midnight one may need to also specify whether it is the midnight at the beginning or the end of the day in question).
History and usage
The 12-hour clock originated in Egypt. However, the lengths of their hours varied seasonally, always with 12 hours from sunrise to sunset and 12 hours from sunset to sunrise, the hour beginning and ending each half-day (four hours each day) being a twilight hour. An Egyptian sundial for daylight use[1] and an Egyptian water clock for nighttime use found in the tomb of Pharaoh Amenhotep I,[2] both dating to c. 1500 BC, divided these periods into 12 hours each. The Romans also used a 12-hour clock: the day was divided into 12 equal hours (of, thus, varying length throughout the year) and the night was divided into three watches.
The Romans numbered the morning hours originally in reverse. For example, "3 a.m." or "3 hours ante meridiem" meant "three hours before noon", compared to the modern meaning of "three hours after midnight".
The first mechanical clocks in the 14th century, if they had dials at all, showed all 24 hours, using the 24 hour analog dial, influenced by astronomers' familiarity with the astrolabe and sundial, and their desire to model the apparent motion of the sun. In Northern Europe, these dials generally used the 12 hour numbering scheme in Roman numerals, but showed both a.m. and p.m. periods in sequence. This is known as the Double-XII system, and can be seen on many surviving clock faces, such as those at Wells and Exeter. Elsewhere in Europe, particularly in Italy, numbering was more likely to be based on the 24 hour system (I to XXIV), reflecting the Italian style of counting the hours.


圖片參考：http://upload.wikimedia.org/wikipedia/en/thumb/1/10/Exetercathedralclock.jpg/200px-Exetercathedralclock.jpg



圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
Exeter Cathedral clock, showing the Double-XII numbering scheme.
During the 15th and 16th centuries, the 12 hour analog dial and time system, with its simpler and more economical construction, gradually became established as standard throughout Northern Europe for general public use, with the 24 hour analog dial reserved for the more specialist applications, such as for astronomical clocks and chronometers.
Today, most analog clocks and watches use the 12-hour dial, where the hour hand (shorter and sometimes thicker) commonly rotates once over the course of every 12 hours, and twice in a day. These are used even in cultures where the 24-hour notation is otherwise preferred. Some 12-hour dials show the numbers 13 to 23 written inside the primary 1 to 12 ring.
Although it has largely been replaced today by the 24-hour notation around the world, especially in written communication, the 12-hour notation with a.m./p.m. suffix is common in some parts of the world.