Is it 2 am or 2 a.m.?

A few countries, namely the United States, Canada, and Australia, use the 12-hour clock instead of the 24-hour clock. This results in a need to differentiate the two halves of a 24-hour period (morning versus night), so the abbreviations “a.m.” and “p.m.” were created. The origin of these abbreviations is simple — “a.m.” stands for the Latin phrase ante meridiem, meaning “before noon,” while “p.m.” is short for post meridiem, the Latin phrase for “after noon.” The 24-hour system, which is used throughout the rest of the world, has no need for such abbreviations.

To answer these questions, we refer to our trusty Chicago Manual of Style.

The following abbreviations are used in text and elsewhere to indicate time of day. Chicago prefers the lowercase form, with periods, as being the most immediately intelligible.

The abbreviations a.m. and p.m. should not be used with morning, afternoon, evening, night, or o’clock.

• 10:30 a.m. /or ten thirty in the morning
• 11:00 p.m. /or eleven o’clock at night

Then what about “o’clock?”

Times of day in even, half, and quarter hours are usually spelled out in text. With o’clock, the number is always spelled out. In the third example, the a before quarter is optional.

• Her day begins at five o’clock in the morning.
• The meeting continued until half past three.
• He left the office at a quarter of four (or a quarter to four).
• We will resume at ten thirty.
• Cinderella almost forgot that she should leave the ball before midnight.

In quotes and conversations in books?

• The author is in charge and makes up the rules. Just try to be consistent.

Hallard Press proofreading policies follow CMoS so that everything is consistent.