What do the abbreviations a.m. and p.m. after hours and minutes mean?

Answer 1

A day is divided into two halves when time is recorded on a twelve hour clock. Since there is a 3:25 in the middle of the afternoon as well as a 3:25 in the middle of the night, it is necessary to indicate which 3:25 is the one being referred to. Therefore, when referring to the period of time after midnight and before noon it is common to add the term "a.m." Likewise, when referring to the period of time after noon and before midnight it is common to add the term "p.m." The letter "m" in each term is an abbrevation of the word "meridian" (which means "noon"). The letter "a" is an abbreviation of the word "ante" (which means before); the letter "p" is an abbreviation of the word "post" (which means "after"). Although 12:00 a.m. is generally considered to be the one at night and 12:00 p.m. the one during the daytime, it is best to clarify 12:00 with the words "noon" or "midnight" in order to avoid the possibility of confusion. Such terms are used only when referring to time measured on a twelve hour clock. When time is measured using a twenty-four hour clock, clarfying time as "a.m." or "p.m." is superfluous.