The full form of am and pm is ante meridiem (before noon) and post meridiem (after noon). The abbreviations “a.m.” and “p.m.” refer to the Latin terms ante meridiem (before noon) and post meridiem (after midday). The abbreviations “a.m.” and “p.m.” can be written in tiny capitals (“am” and “pm”), uppercase letters without a period (“AM” and “PM”), uppercase letters with periods, or lowercase letters, depending on the style guide (“am” and “pm” or, more commonly, “a.m.” and “p.m.”). The “M” character is commonly deleted from computer-generated and printed timetables, especially for airlines, because it provides no further information, as in “9:30A” or “10:00P.”
A space between the number and the a.m. or p.m. abbreviation is recommended by several stylebooks. Although using a.m. and p.m. without a time before them is discouraged by style guides, it can be useful when describing an event that always occurs before or after noon.
Different countries use different hour/minute separators: some use a colon, some use a period (full stop), and yet others use the letter h. There is no separator between hours and minutes in many cases when utilizing a 24-hour clock (0800, read as written, i.e. “zero-eight-hundred” or more commonly substituting the letter O for the numeral zero, as “oh-eight-hundred”).