These are abbreviations for ante meridiem ( a.m. ), meaning before mid-day, and post meridiem ( p.m. ), meaning after mid-day. They should be set in lowercase, small caps, with periods and no spaces; also, lower the type size by one point (for contrast). Avoid redundancies like 3:15 a.m. in the morning, and an afternoon break at 2:30 p.m. See time of day.
Dates should not be abbreviated in running text or body copy. However, in tables and other instances where space is limited, dates may be abbreviated with numerals and slashes:month[slash]two-digit day[slash]four-digit year.
- The symposium is scheduled for February 3, 2005.
- [abbreviated entry:] 2/03/2005
Also, do not use ordinal numbers in dates (see days of the month, below).
- December 5, not December 5th
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Days of the month
Days of the month are treated as cardinal numbers unless the month is not mentioned.
- The next Web site meeting is scheduled for the morning of May 12 at 10:00.
- Sean will be seeing the doctor again about his injured ankle on the 5 th.
Days of the week
Do not abbreviate days of the week in running text. Where space is tight, as in tables or outlines, abbreviate as follows:
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Decades and centuries
Decades and centuries can be spelled out, or numbers with apostrophes can be used. There is never an apostrophe between the number and the letter s.
- the 1990’s
- the seventies and the eighties
- the 1970s
- the 1800s
- the ’70s and the ’80s
References to a century will be hyphenated or not depending on whether it is a noun or adjective: 20th century (noun); 20th-century (adjective).
- Some folks were happy to see the 20th century come to a close.
- Elizabeth ’s specialty is 20th-century literature.
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Lists of times
When listing times, for example of performances, give the minutes for all of them if you must give the minutes for any.
- He’s free for 20-minute conferences on Thursday at 9:20, 1, and 3:40.
- He’s free for 20-minute conferences on Thursday at 9:20, 1:00, and 3:40.
Months should not be abbreviated in running text or body copy. However, in tables and other instances where space is limited, the longer months may be abbreviated as follows. Normally, May, June and July are never shortened.
Always spell out months when used alone or with a year.
- He arrived on July 10 and left on August 12.
- The next IAP is in January 2005.
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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.
Spans of time (or numbers)
It is preferable to use from … to or from …through where spans of dates, days of the week or times are concerned.
- The party is from 8–11 p.m. [Note that from is incorrectly combined with an en dash.]
- The party is from 8 to 11 p.m.
- The geography class meets Mondays through
- Thursdays from 8 to 10 a.m.
However, if abbreviations are necessary due to space constraints, use an en dash.
- Madison is a regular Monday–Friday volunteer at the library.
In time spans involving years, use the four-digit year/en dash/two-digit year format unless the years span a century. In the latter case, all four digits must always be used for both years.
- The Student Government Association approved new rules during the 2003–4 academic year.
- Her final year of teaching was 1999-00.
- The Student Government Association approved new rules during the 2003–04 academic year.
- Her final year of teaching was 1999-2000.
- The brain and cognitive sciences project is scheduled to be completed in 2005–06.
For both years and numbers, when the first number ends with more than one zero, the complete number should follow.
- Look for the information on pages 100–2.
- The information can be found on pages 100–103.
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Time of day
Time of day is generally expressed in numerals followed by a.m. or p.m. with the exception of noon and midnight. In those cases. eliminate confusion by specifying “12:00 noon” or “12:00 midnight” rather than 12 a.m. or 12 p.m.:
- The Seinfeld marathon starts this Saturday at 12 a.m.
- The Seinfeld marathon starts this Saturday at 12:00 midnight.
Also again with the exception of noon and midnight, when 12:00 noon or 12:00 midnight is called for unless the exact minutes are needed, times can be expressed without the :00, as follows: 3 a.m., 7 p.m., 4:30 p.m. (note spacing and use of periods).
Times are only written out in very formal circumstances, such as for invitations, and then the written number is often accompanied by the word o’clock:
- President Hockfield requests the pleasure of your company at Gray House at half-past seven o’clock.
- Half-past seven o’clock, not 7:30 o’clock
- The party is from 8 to 11 p.m., not The party is from 8 to 11 p.m. tonight.
See a.m., p.m.
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