Use numerals rather than words to express the time of day in 12-hour format, and use a colon to separate the minutes from the hours.
You do not need the extraneous zeros to the right of the colon for the straight-up hour. Use lower case letters and periods with no space in between for the abbreviations for ante meridiem (Latin for “before midday”) and post meridiem (“after midday”).
10 a.m. 3 p.m. 9:45 a.m. 2:45 p.m.
The construction “3 o’clock in the afternoon” is acceptable, but it is usually only used in formal invitations.
Here are some Bad Examples and Corrected Examples:
eight p.m. => 8 p.m. (use numeral, not word)
9 a.m. this morning => 9 a.m. today (redundant)
8:30 p.m. Tuesday evening => 8:30 p.m. Tuesday (redundant)
Is noon 12 a.m. or 12 p.m.?
Is midnight 12 p.m. or 12 a.m.?
Neither. They are simply noon and midnight. They are not 12 noon or 12 midnight.
According to the AP Stylebook, midnight is considered part of the day that is ending, not part of the day that is beginning, contrary to the practice of most New Year’s revelers.
Airlines and railroads use 12:01 a.m. for arrivals and departures at the very beginning of the day, and they use 12:01 p.m. for noontime trips.
The easy way to remember this is p.m. = post meridiem, or immediately after midday.