This is a stylistic minefield and the most important aspect to remember is to be consistent throughout your document and in all the documents for a particular client. If you are ever asked to use a specific style guide, then this is an area you’ll definitely need to look up because there are so many variations. This post is going to focus on the recommendations in the New Oxford Style Manual and compare them with those found in The Chicago Manual of Style, so essentially the difference between UK and US usage.
About a.m. and p.m.
The first bit of good news is that if you use the 24-hour clock, you won’t have to bother with a.m. (ante meridiem) and p.m. (post meridiem).
You can either write 10.40 p.m. or 22.40
But if you only mention the hour, you should write 10 p.m. and not 10.00 p.m. (i.e. lose the zeros) according to Oxford. Chicago, however, recommends the opposite.
10 p.m. = correct for UK, incorrect for US
10.00 p.m. = incorrect for UK
10:00 p.m. = correct for US (note the colon, see next section below)
However, if you do use a.m. and p.m., you do still need those points according to Oxford, so 10.40 am isn’t correct. But neither is 10.40a.m. as you need to leave a space between the number and the abbreviation (but other style guides might tell you a different story). Chicago recommends the same as Oxford, but acknowledges that they can also sometimes be written without the points or in small capitals.
Should I use points or colons with the time?
In UK English the number is written with a point. In US English, according to Oxford, it’s a colon. I cannot find where this is specified in Chicago, but all the examples I’ve seen in this book use a colon.
10.40 p.m. = correct for UK (point, no colon)
10:40 p.m. = correct for US (with a colon)
10.40p.m. = incorrect (a space is needed between the time and the abbreviation)
10.40 pm = incorrect (points are needed in the abbreviation)
10.40pm = incorrect (points are needed in the abbreviation and a space is needed between the time and the abbreviation)
Twenty-four hour system
As far as Chicago is concerned, time written using this system can drop all punctuation. Oxford doesn’t specify as such, but the examples given have a point.
2210 is perfectly possible
22.10 would appear to be the Oxford preference
22:10 would appear to be the Chicago preference
2210 hours (and variations. i.e. 22:10 hours and 22.10 hours) and 2210h (and variations) are also all possible.
As mentioned above, you need to make sure you choose just one way of writing the time in your document and then stick to this choice throughout.
If you want to express seconds as well as minutes, the first full point disappears according to Oxford.
0723.58 hours (twenty-three minutes past seven o’clock in the morning and 58 seconds)
But according to Chicago, the first colon does not disappear.
07:23:58 (twenty-three minutes past seven o’clock in the morning and 58 seconds)
The twelve o’clock problem
Misunderstandings could arise if you use 12 a.m. (or 12:00 a.m.) and 12 p.m. (or 12.00 p.m.) and as a result Chicago categorically states that they should not be used.
12 a.m. = midnight, but you can also say 12 midnight or use the 24-hour clock and write 24.00 to avoid confusion.
12 p.m. = midday or noon, so as above you can say 12 noon or 12 midday or use 12.00 to make the time clearer.
Writing the time in words
Of course you can also say ten o’clock, i.e. write the time out in words, and this is often preferred when no minutes are involved. Expressing 15, 30 and 45 minutes past the hour are also often written in full, e.g. quarter past ten, half past ten and quarter to nine. Please note that there are no hyphens and o’clock is used only with the exact hour when the number is written in full.
10 o’clock = incorrect. It should be ten o’clock (according to both Oxford and Chicago)
quarter past ten o’clock = incorrect. It should be quarter past ten
half past ten o’clock = incorrect. It should be half past ten
Use a.m. and p.m. with numbers, as in the examples above, but not when the number is written as a word.
ten o’clock in the morning = correct
10 a.m. = correct UK
10:00 a.m. = correct US
ten o’clock a.m. = incorrect
ten o’clock at night = correct
10 p.m. = correct UK
10:00 p.m. = correct US
ten o’clock p.m. = incorrect
This post was first published on 02/04/2015 on my previous blog.