Writing the Time: AM and PM or a.m. and p.m.?

AM and PM are terms used when writing times. However, there are a few different ways to write them, which can be tricky when proofreading. Should you just aim for consistency? Or is there a correct way to do it in certain contexts? In this post, we explore your options.

What Do These Terms Mean?

“AM” and “PM” are both abbreviations of Latin terms and refer to a specific time of day:

  • AM (ante meridiem) means “before noon,” so it refers to the morning.
  • PM (post meridiem) means “after noon,” so it refers to any time after midday.

When using a 12-hour clock, then, these terms clarify the time we have in mind (e.g., 12 AM is midnight, whereas 12 PM is midday). This is not necessary when using a 24-hour clock.

There are, however, a few options for how you write these terms, as we will explore below.

Capitalization of AM and PM

You may have noticed that we write “AM” and “PM” with capital letters in this post. For instance:

The performances today will be at 11 AM and 3 PM.

But that is not the only legitimate option! You can also write them with lowercase letters:

The performances today will be at 11 am and 3 pm.

Or you can write them with small caps. All are acceptable ways of writing these terms, so ultimately it comes down to preference or your client’s chosen style guide.

Punctuation of AM and PM

Another variation is adding periods between each letter in these terms. For instance:

The meeting ended at 11 a.m.

Did you see the 5:00 p.m. bulletin?

This is common when using lowercase letters, although some also add punctuation when using small caps. It would be unusual to punctuate these terms when using standard capital letters, though. Finally, keep in mind that you do not need an extra period after the abbreviation when punctuating a time like this at the end of a sentence.

Spacing AM and PM

The final variation you will see with these terms is whether to add a space between the time and AM/PM. In the examples so far, we have added this space, as it is more common to do so.

However, when using the unpunctuated forms of these terms, you can close the gap. This does not apply when using the punctuated forms, so take care not to mix up these styles:

Spaced and Unpunctuated: I have a class at 2 PM today.

Unspaced and Unpunctuated: I have a class at 2PM today.

Spaced and Punctuated: You need to be there by 9:00 a.m.

Unspaced and Punctuated: You need to be there by 9:00a.m.

As with the other options here, the space before AM and PM is largely a matter of preference.

What Do Style Guides Recommend?

As you can see, there are many options available when writing these terms. And as a proofreader, your main priority will be to ensure your client uses a consistent style. However, if your client is using a style guide, too, you can check it for advice.

Some well-known style guides suggest the following:

Style GuideLowercase or Capital?Punctuation?Spacing?
AMASmall capsNoYes
APALowercaseYesYes
Associated PressLowercaseYesYes
ChicagoEither lowercase or small capsYes with lowercase letters; optional with small capsYes
MHRA Lowercase NoNo
MLA Lowercase YesYes

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Join the Conversation

Rose says:
February 23, 2021 at 4:57AM
While writing AM and PM, you have to use punctuation with small case and not with capitals.
    Proofreading Academy says:
    February 23, 2021 at 9:50AM
    Hi, Rose. Thanks for the comment. We've updated the section in question for clarity.
Martin says:
April 13, 2021 at 9:44PM
interesting to see the time stamp of the comments on this website showing as "Am" and the comments forcing initial caps here in this section.
    Proofreading Academy says:
    April 14, 2021 at 8:25AM
    Hi, Martin. Thanks for pointing these out. Hopefully we can fix both soon.
James Currie says:
April 28, 2021 at 5:18PM
Is it proper to refer to noon and midnight with a.m. or p.m., or should you just say 1200 noon and 1200 midnight?
    Proofreading Academy says:
    April 29, 2021 at 7:34AM
    Hi, James. You don't need the "12" at all when using "noon" and "midnight" (it would be a redundancy, since "noon" and "midnight" automatically imply being 12 p.m. and 12 a.m., respectively). As such, you don't need the "a.m." or "p.m." either. And even if you did want to use "12" with either word, you wouldn't need the abbreviation as well (as above, the words "noon" and "midnight" would already make it clear what time of day you're referring to, so "a.m." and "p.m." wouldn't give any extra information).
mike says:
June 24, 2021 at 9:53PM
AM (ante meridiem) means “before noon,” so it refers to the morning. PM (post meridiem) means “after noon,” so it refers to any time after midday. So how is that 12:00 noon(midday) is called 12pm when it is neither before or after midday. I would posit that 12:00 midnight is neither am or pm as that it is equally both. So the proper term is midday/noon or midnight. or properly written it would be 12:01 pm or 11:59 am if you wish to use the am pm method of delineation for midday. Otherwise it needs to be described as the common usage is 12:00 pm refers to midday.
    Proofreading Academy says:
    June 25, 2021 at 9:04AM
    Hi, Mike. I think the common usage is clear enough as it is, isn't it? If nothing else, your concern about the exact moment of 12pm being neither before nor after midday would last less than a second before we're officially into the afternoon!

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