A.M., P.M., a.m., p.m.: How to Write Time

By
Lynn Gaertner-Johnston

Let’s discuss the proper way to write time, whether standard, military time or the 24-hour clock. To start, let’s discuss standard 12-hour clock that uses a.m. and p.m. Two commonly seen formats are 4pm and 4:00pm. Which is correct?  The answer is: neither!

Both renderings are incorrect. You need to insert periods and a space before the abbreviation, like this:

4 p.m. or 4:00 p.m.

Graphic illustrating "what is the correct time." A general rule of thumb to keep in mind is whatever style you choose, be consistent.

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:
4 P.M. (However, Microsoft prefers 24-hour time notations, in which 4 P.M. is 16:00.)

The Chicago Manual of Style 
4 p.m. (recommended)
Also 4 PM or 4 P.M.(with PM in small capitals)

Garner’s Modern English 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. The author’s preference is to always use lowercase letters: 4 p.m.

Omit zeros when the time is on the hour (unless you want to emphasize the time precisely), 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

To make sure you are never doubtful about how to write time, date or any other formatting question, we recommend investing in a style manual.

What do a.m. and p.m. stand for?

While we are on the topic, what do these abbreviations stand for? 

Well, a.m. stands for the Latin “ante meridiem” – before midday, while p.m. stands for “post meridiem” – after midday. Both refer to the sun’s positions in relation to the meridian. 

A graphic explaining the meaning of a.m. (Latin for ante meridiem, or "before midday") and p.m. (post meridiem or "after midday")

What is military time? How does it work?

Now let’s discuss how to write military time. Military time is a variation of the 24-hour clock and is a way to talk about time using 24 hours instead of 12 hours. When using the 24-hour clock time, the day begins at 0 hours and ends in 24 hours. 

24-hour clock is widely used across the globe, except in a few countries (notably the United States), which include:

  • Canada
  • Mexico
  • the Philippines
  • Egypt
  • Australia

In the United States, and the countries listed above, we use a 12-hour clock, which splits into two blocks of time: 

First block: midnight (12:00 a.m.) to noon (12:00 p.m.)

Second block: noon (12:00 p.m.) to midnight (12:00 a.m.)

24-hour clock time is displayed in four digits, just like a 12-hour digital clock. The first two digits represent the hour, while the last two digits represent the minutes. 

00:00 is pronounced “zero-hundred” and represents midnight, while 1200 is pronounced “twelve-hundred” and represents noon. 

Anything between 00:01 and 11:59 is “a.m.” 

Anything between 12:01 and 23:59 is “p.m.” 

For example, 0200 (pronounced” zero, two-hundred”) converts into 2 a.m., while 1600 (pronounced “sixteen-hundred”) converts into 4 p.m.

Military time vs. 24-hour clock

There is a slight difference between military time and the 24-hour clock:

  • Military time does not use a separator between the hour (the first two digits) and the minutes (the last two digits).

Military: 2200

24-hour clock: 22:00

  • Military time pronounces the leading zeros (i.e., 0300 is pronounced “zero three hundred.”
  • In military time, the word “thousands” is not used. 1000 can not be pronounced “one thousand.” Instead, it is pronounced one of the following ways:

“Ten hundred”

“Ten zero zero”

“One zero zero”

Who uses the 24-hour clock format?

In the United States, the 24-hour clock is used in various specialist areas, such as:

  • Astronomy
  • Aviation
  • Computing
  • Emergency services
  • Logistics
  • Meteorology
  • Navigation
  • Tourism

Military time to regular time chart

MILITARY TIME – STANDARD TIME

0100 – 1:00 AM

0200 – 2:00 AM

0300 – 3:00 AM

0400 – 4:00 AM

0500 – 5:00 AM

0600 – 6:00 AM

0700 – 7:00 AM

0800 – 8:00 AM

0900 – 9:00 AM

1000 – 10:00 AM

1100 – 11:00 AM

1200 – 12:00 PM || NOON

1300 – 1:00 PM

1400 – 2:00 PM

1500 – 3:00 PM

1600 – 4:00 PM

1700 – 5:00 PM

1800 – 6:00 PM

1900 – 7:00 PM

2000 – 8:00 PM

2100 – 9:00 PM

2200 – 10:00 PM

2300 – 11:00 PM

2400 or 0000 – 12:00 AM || MIDNIGHT

Further reading: Commas in Dates: Writing a Date Correctly

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Lynn Gaertner-Johnston
Lynn Gaertner-Johnston has helped thousands of employees and managers improve their business writing skills and confidence through her company, Syntax Training. In her corporate training career of more than 20 years, she has worked with executives, engineers, scientists, sales staff, and many other professionals, helping them get their messages across with clarity and tact. A gifted teacher, Lynn has led writing classes at more than 100 companies and organizations such as MasterCard, Microsoft, Boeing, Nintendo, REI, AARP, Ledcor, and Kaiser Permanente. Near her home in Seattle, Washington, she has taught managerial communications in the MBA programs of the University of Washington and UW Bothell. She has created a communications course, Business Writing That Builds Relationships, and provides the curriculum at no cost to college instructors. A recognized expert in business writing etiquette, Lynn has been quoted in "The Wall Street Journal," "The Atlantic," "Vanity Fair," and other media. Lynn sharpened her business writing skills at the University of Notre Dame, where she earned a master's degree in communication, and at Bradley University, with a bachelor's degree in English. She grew up in suburban Chicago, Illinois.

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52 COMMENTS

  2. 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

  3. 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.

  4. 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

  5. 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.

  7. 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.

  9. 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

  10. I also write AM and PM without the periods or the spaces. I always thought that writing A.M. or a.m. or P.M. or p.m. was not the correct way of writing that. I guess I am an old stick in the mud, and since I hate change, or newer ways of doing things, I am going to stick with writing AM and PM. It might be wrong, or grammatically incorrect, but it is what I am used to.

  11. Christine, you are not an old stick-in-the-mud. You are on the cutting edge! Only some style manuals have begun to leave out the periods in question. My “Handbook of Business English,” first published in 1914, includes them.

    You didn’t know how modern you are.

    Lynn

  12. Is it proper to write: At 4p.m. this afternoon we will be meeting. . .
    Is it improper to add the word ‘afternoon’ if you have stated that the time is 4 p.m.?

  13. Hi, Ben. It is redundant to write “4 p.m. in the afternoon.” However, “4 p.m. this afternoon” does clarify which afternoon (for example, not tomorrow afternoon).

    A better choice is “4 p.m. today.”

    Lynn

  14. Any answer to:
    “What about a period of time? If a function runs from 5 p.m. – 7 p.m. Or is it 5 – 7 p.m.?”

  16. What is the correct way to write 12 midnight or 12 noon

    Is the 12 noon, 12:00p.m. or 12:00 a.m.?

  17. Sally, many reference books recommmend simply “midnight” or “noon.” However, if you are entering that time with a list of other times, use “12 midnight” or “12 noon.” Using the number alone with a.m. or p.m. can be confusing in some contexts, so avoid that approach.

    Lynn

  18. Hi Lynn,

    I’ve been searching online for the proper way to list an event day, date and time and came across this blog post. Perhaps you can help? Is the following the correct way:

    XYZ panel takes place on Tuesday, September 13, at 11:00 a.m.

    Thanks in advance for any clarification!

  22. Dear Lynn,
    How would I write a time when asking a question?

    Example: Are you available at 4p.m.?

    Would I have the periods between the “p” and the “m” and then put the question mark?

  31. I question the requirement of spaces before the am/pm, for modern practicality uses. Not everyone knows how to use non-breaking spaces, and if that isn’t used, then, particularly on websites and in Email where one cannot guarantee the same fonts, browser width (especially relevnt for fluid layouts, which are coming back into style due to smartphones), etc., the possibility exists of word-wrap causing the time to appear at the end of a line, and the am/pm at the beginning of the next line, like so:

    “The fun starts at 7
    p.m. and lasts until
    midnight!”

    This cannot happen if you use, for instance, “7pm”.

    Of course, the best solution is to just learn to use non-breaking spaces as needed.

  33. a.m. and p.m. creates doubt in some contries. i suggest it will be better to use 24 hours clock time.so no doubt is there isn`t it?

  35. If you are writing the time range 11:00 a.m. – 1:00 p.m., do you have to include the a.m., or is it okay to write 11:00-1:00 p.m.?

  37. I can’t find much support for my way of thinking, but using lowercase letters and periods just looks “old school” to me. I’ve worked as a tech writer for more than 30 years (much of it in IT), and it just looks cleaner to omit the periods.

    “The upgrade is scheduled for 7:00 am tomorrow.”

    “The upgrade is scheduled for 7:00 pm tomorrow.”

    I agree that when used in a sentence, the morning designation without periods COULD be read as the word “am,” and yet I don’t worry about this since the context is clear.

    That’s my opinion, and I’m stickin’ to it! 🙂

  38. Hi, Kathy. You are correct that reference manuals don’t agree with you, not even “Microsoft Manual of Style,” which was published this year.

    I felt the same way about the word “gray.” I wanted to spell it “grey,” the British way, because it looked right to me. But living and working in the US, I decided to give up my preference and use “gray.”

    Good luck with your choice.

    Lynn

  39. You would think finding this information would be relatively simple, straight-forward and easy, given the amount of technology at our fingertips. However, I’ve been searching for nearly 15 minutes on this, which is really too much time, and I’m still frustrated. I have the St. Martin’s Guide to Library and Research Documenting, which lists AMA, APA, CBE, and Note-and-Bibliography styles for just about everything, but it also lists abbreviations for time, acronyms, geographical names, and a few other things that are shared amongst all styles. The proper format for time is lower case with periods. It also explains that a.m. is ante meridiem, “before noon” and p.m. is post meridiem, “after noon”. This makes sense as to why the periods are necessary, though I don’t know how consistent it is across all Latin translations like that. For example, “for example” (<--haha!) we use "e.g.", but for "and others" we use "et al." and not "et. al." Anyway, I'm going to go with lower case and periods even though it's a pain. And as for gray/grey- I realize that after I read that, I actually search it every time I write it because I'm never sure. I'm not even sure if I search grey or gray, but since both ways are correct, whichever one I search comes up so I use it and assume it's right. That is probably the reason I never know which is right because they are both right! (I have no idea if that last part made sense or not.) 🙂

  40. Hello, AC. Life is full of complexity, isn’t it?

    I am happy to tell you that the “et” in “et al.” has no period because it is not an abbreviation. The phrase when spelled out is “et alia.”

    Regarding “grey” and “gray,” “gray” is preferred in the United States. I am not certain which is preferred in other countries.

    Thank you for stopping by.

    Lynn

  41. Hi Lynn,

    What is the correct order/style when you write a sentence which contains following parts: time, day, month, year, day of the week? What is the correct order of those parts? Thanks!

    zky

  42. Zky, here is an example to answer your question:

    I will see you on Wednesday, November 28, 2012, at 11 a.m.

    OR

    I will see you at 11 a.m. on Wednesday,
    November 28, 2012.

    You do not need to include the year if it is obvious and you are not writing a legal document. In both examples above, you would simply delete the year.

    Lynn

  43. Lynn,

    Thank you for this great post! My only question is how do I use proper punctuation with the examples shown. If a sentence ends with the time, do I need to insert a period after the “p.m.”?

    For example: I have to meet Cathy at 2 p.m.

    I assume in this sentence, I would not insert a period after the time because that would look silly (2 p.m..).

    However, if I write this same sentence as a question, I would obviously insert a question mark.

    Do I have to meet Cathy at 2 p.m.?

    Could you please clarify the rule on punctuation when the time is written at the end of a sentence?

    Jared

  47. The first comment to this post, posted on June 06, 2009 at 04:47 AM, and the most recent, December 30, 2012 at 09:46 AM.

    Ironically, for over three years the time format for this blog has been: Month Day, Year at 00:00 AM or Month Day, Year at 00:00 PM.

    It seems the only .s being used these days are at the end of sentences. My child, at times, avoids them there as well. 🙂 or 🙁

  48. Hi, Randy. Woe is us! (Woe is WE?)

    When I get frustrated over a lack of punctuation, I try to remember that I can control only my own behavior, not that of others.

    Good luck with your child!

    Lynn

  51. When I proofread printed material at my office, I always make the times conform to the format you have suggested (4:00 a.m.). Recent college graduates I work with accuse me of being from another planet for my preference. Since part of our operation is an AM radio station, I get some mileage out of the need to make 11:20 a.m. look different from 1120 AM. Now, don’t get me started on using periods, instead of hyphens and parenthesis, in phone numbers (like 800.555.1234).

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