A.M., P.M., a.m., p.m.: What Is the Correct Time?

Pam wrote to me to settle a debate between the younger and the older employees in her office. Her brief question was this:

4pm or 4:00pm?

My answer: Neither!

Both of Pam's renderings are incorrect. She needs to insert periods and a space before the abbreviation, like this:
4 p.m. or 4:00 p.m.

In email, many people seem to be dropping both the periods and the space, but don't follow the crowd. The prominent style guides do not support that choice. Here is a sampling of recommendations:

The Associated Press Stylebook:
4 p.m.

Microsoft Manual of Style for Technical Publications:
4 P.M.

The Chicago Manual of Style and Garner's Modern American Usage:
4 p.m. or 4 PM (with PM in small capitals)

The Gregg Reference Manual:
4 p.m. or 4 P.M. (with PM in small capitals)

Whatever style you choose, be consistent. I always use lowercase letters: 4 p.m.

Pam's coworkers were apparently arguing about whether to include the zeros. Omit zeros when the time is on the hour, but include them in a list of varying times like this one:

7:00 a.m. Registration
7:30 a.m. Breakfast
8:00 a.m. Announcements
8:15 a.m. Speaker

Pam, it's time for you and your coworkers to pitch in and get a style manual. See the recommendations on my website.

Does your style guide recommend a different approach to time? If so, please share it here.

Lynn
Syntax Training

Comments

[Srikanth]

Great, never thought about it before. Thank you!

[Conrado Calvet]

I prefer use the 24 hr clock for schedulling, it´s a good option and it´s easier avoid misunderstadings for the people whom just pass a eye in the schedule.

Why dou you think Lynn?

Kind Regards
Conrado

[Lynn]

I think the 24-hour clock is an excellent idea. I find, though, that many people in the United States prefer a.m. and p.m., particularly if their business does not run 24 hours a day.

[Lisa]

My style manual is the American Medical Association's Manual of Style, which avoids using punctuation in abbreviations (up to and including MD, eg, and ie). It's one of the hardest things for people to get used to when they start writing at my company. The other hard thing for people to remember is not spelling out numbers unless they start a sentence -- even numbers under 10! You can see a brief overview of AMA style here:
http://www.docstyles.com/archive/amastat.pdf

[Lynn]

Lisa, thanks so much for sharing the information and a resource. It is helpful to know about the differences.

I can understand how new employees struggle with the punctuation and number rules. I am glad you have a style sheet to support the new writers--and the rules.

[Sue]

What about a period of time? If a function runs from 5 p.m. - 7 p.m. Or is it 5 - 7 p.m.?

[John]

Actually, in a business context, it is best to ALWAYS use zeros (4:00). Though this rule can "bend" for native speakers, if you are working with internationals, it's better to have a consistent rule that is applied at all times.

Also, when using the 24-hour clock (a norm for those outside North America), you should use a leading zero to avoid confusion (04:00 rather than 4:00).

Finally, it's important to avoid being draconian when writing emails. A new register has actually been created for emails: neutral. So there is formal for letters and legal/offical matters, informal for person correspondence and neutral for business email correspondence.

[dyhlon]

What about if the 4 P.M. is at the end of the sentence? Do you need to put another period?

[Lynn Gaertner-Johnston]

Sue, one mention of "p.m." is sufficient when your meaning is obvious.

John, thank you for mentioning international audiences and their needs.

Dyhlon, you need only one period at the end of the sentence. Two periods would be incorrect.

Lynn