Use uppercase letters, no periods, a single preceding non-breaking space character, and the small tag to specify ante meridiem or post meridiem with times of day.
INCORRECT:
3:30pm, 3:30PM, 3:30 p.m., 3:30 P.M., 3:30 pm, 3:30 PM
CORRECT:
7:15 AM
and
3:30 PM
7:15 <small><abbr title="ante meridiem">AM</abbr></small>
3:30 <small><abbr title="post meridiem">PM</abbr></small>
When specifying an event occurring at 12:00 noon or 12:00 midnight, explicitly state noon or midnight to remove the ambiguity of what AM and PM mean when paired with 12:00.
You can use MID as an abbreviation for MIDNIGHT, but as with the style above, use all caps and no periods set in a small tag, preceded by a single non-breaking space character.
INCORRECT:
12:00 NOON, 12:00 N, 12:00 AM, 12:00 PM
CORRECT:
12:00 NOON, 12:00 MID, 12:00 MIDNIGHT
12:00 <small>NOON</small>
12:00 <small><abbr title="midnight">MID</abbr></small>
12:00 <small>MIDNIGHT</small>
We use this style because it is more aesthetically pleasing on computer displays than any other style, and it conserves valuable screen space by omitting the needless periods.
Related Topics: Date and Time Format, Time Zone, Daylight Savings Time.
Go To: Usage Details or Content Style or Style Guide