Just a warning about noon and midnight: one should never use a.m. or p.m. (or ᴀᴍ and ᴘᴍ)
in conjunction with noon and midnight. They are at best ambiguous and confusing, and at worst, simply wrong. Noon is neither ᴀᴍ nor ᴘᴍ,
and midnight is arguably both.
You can also easily find conflicting opinions about whether noon is 12 ᴀᴍ or 12 ᴘᴍ; fifty years ago it was more likely that noon was considered 12 ᴀᴍ and midnight was 12 ᴘᴍ, but today it is more common to see those expressed the other way around.
But both are wrong, so this will get you in trouble.
For example, a court case in Ocean Cit, Maryland came up because the parking meters said they were active during “8 ᴀᴍ – 12 ᴘᴍ”. Someone fought a ticket they’d gotten after noon, since they thought it meant that the meters stopped at noon but the city thought it stopped at midnight. The judge ruled against the city for putting up a confusing and ambiguous notice, and all meters there now read “8 ᴀᴍ – midnight” instead.
For reasons of correctness, confusion, and ambiguity, most style guides tell you to never use ᴀᴍ or ᴘᴍ with noon and midnight, and that you should instead write either 12 noon and 12 midnight, or else dispense with the numbers altogether and simply write noon and midnight.
Authoritative references for this and excerpts from many English-language style guides follow.
According to both the National Institute of Standards and Technology FAQ and the National Physical Laboratory FAQ alike, there is no such time as “12 p.m.” or “12 a.m.”, and these should not be used. Instead, “12 noon” and “12 midnight” should be used.
Specifically, NIST says:
Are noon and midnight referred to as 12 a.m. or 12 p.m.?
This is a tricky question because 12 a.m. and 12 p.m. are ambiguous and should not be used.
[. . .]
Hours of operation for a business or other references to a block of time should also follow this designation rule. For example, a business might be open on Saturdays from 8 a.m. to noon or weekends from 3:30 p.m. until midnight.
And NPL states:
Is midnight 12 a.m. or 12 p.m.? (FAQ - Time)
There is no confusion when using the words 12 noon (or mid-day) and 12 midnight, although the use of 12 midnight can raise the question of 'which day?'. To avoid confusion in, for example, an insurance certificate, it is always better to use the 24-hour clock, when 12:00 is 12 noon and, for example, 24:00 Sunday or 00:00 Monday both mean 12 midnight Sunday/Monday. It is common in transport timetables to use 23:59 Sunday or 00:01 Monday (in this example), or 11:59 p.m. or 12:01 a.m., to further reduce confusion.
[. . .]
Another convention sometimes used is that, since 12 noon is by definition neither ante meridiem (before noon) nor post meridiem (after noon), then 12 a.m. refers to midnight at the start of the specified day (00:00) and 12 p.m. to midnight at the end of that day (24:00). Given this ambiguity, the terms 12 a.m. and 12 p.m. should be avoided.
I can attest to seeing 12 p.m. used for midnight of one day and 12 a.m. for that same midnight but of the next day.
And here from the time FAQ from the National Maritime Museum at Greenwich (people who should know about time :)
Is noon 12 a.m. or 12 p.m.?
12 noon is neither a.m. nor p.m.
To avoid confusion, the correct designation for 12 o'clock is 12 noon or 12 midnight. Alternatively, the 24-hour clock system may be used.
The abbreviation a.m. stands for ante-meridiem (before the Sun has crossed the line) and p.m. for post-meridiem (after the Sun has crossed the line). At 12 noon, the Sun is at its highest point in the sky and directly over the meridian. It is therefore neither 'ante-' nor 'post-'.
The Middlebury College Condensed Style Guide reads:
Always use “noon” and “midnight” instead of 12:00 a.m. or 12:00 p.m. There is no such thing as 12:00 a.m. or 12:00 p.m. because a.m. begins immediately after midnight and p.m. begins immediately after noon.
The AP Style Guide reads:
Time is always written as a numeral attached to a.m. or p.m., e.g., 7 a.m., 2 p.m., 8:15 a.m., 9:21
p.m. If you choose to write the time as 5 o'clock, you do not write 5 o'clock p.m.; you write 5
o'clock in the morning or evening. There is no such thing as 12 a.m. or 12 p.m.; use midnight and
MIT’s ComDor Editorial Style Guide’s section on “Date and time terminology” reads:
Noon and midnight
Do not use 12 a.m. or 12 p.m., as they are ambiguous. Instead, 12 noon or 12 midnight or, better yet, simply noon or midnight.
More from our friends at Greenwich:
A.M. and P.M.
What is Noon and Midnight?
A.M. and P.M. start immediately after Midnight and Noon (Midday) respectively.
This means that 00:00 A.M. or 00:00 P.M. (or 12:00 A.M. and 12:00 P.M.) have no meaning.
Every day starts precisely at midnight and A.M. starts immediately after that point in time e.g. 00:00:01 A.M. (see also leap seconds)
To avoid confusion timetables, when scheduling around midnight, prefer to use either 23:59 or 00:01 to avoid confusion as to which day is being referred to.
It is after Noon that P.M. starts e.g. 00:00:01 ᴘᴍ (12:00:01)
From McMaster University’s writing style guide:
DATES AND TIMES
12 noon and 12 midnight (rather than 12 a.m. or 12 p.m.)
From Columbia University Chicago’s Editorial Style Guide:
a.m., p.m. — Numerals are used when exact times are emphasized. Use lowercase and periods. (5:22 a.m., 2:53 p.m.). Do not use ᴀᴍ, ᴘᴍ,
or am, pm. Numbers should never be used to express noon or midnight. (In other words, do not use 12 p.m. or 12 a.m.) Do not use 12 noon or 12 midnight.
From the Buttler University Invitation Style Guide:
Time of Event
- Include the minutes unless the time is the top of an hour (e.g., 10:30 a.m., 10 a.m.).
- Always include a.m. or p.m. unless the time is noon or midnight. Write a.m. and p.m. lowercase with periods.
- Do not use 12 with noon or midnight.
From the University of Southern Indiana style guide:
From the Hull University style guide:
time of day Use numerals in ‘8 am’, ‘11.30 pm’, etc; spell out the hour in ‘eight
o’clock’, ‘half past eleven’, etc. Never mix the two conventions: do not write, for
example, ‘7 o’clock’, ‘seven pm’, or – even worse – ‘7 o’clock pm’. Never use ‘am’ with
‘morning’ or ‘pm’ with ‘evening’ (that is, do not write ‘9 am in the morning’ or ‘7 pm in
the evening’), and never use the forms ‘12 am’ and ‘12 pm’ (for noon and midnight).
From The Guardian’s style guide:
1am, 6.30pm, etc; 10 o'clock last night but 10pm yesterday; half past two, a quarter to three, 10 to 11, etc; 2hr 5min 6sec, etc; for 24-hour clock, 00.47, 23.59; noon, midnight (not 12 noon, 12 midnight or 12am, 12pm).
There are plenty more examples out there containing the advice to never use ᴀᴍ and ᴘᴍ with noon and midnight. Using ᴀᴍ and ᴘᴍ with 12 o’clock will always risk getting you in trouble. Don’t do it. Write noon and midnight if you are on a 12-hour clock, or avoid the whole mess entirely by using a 24-hour clock.