Months

When a month is used with a specific date, abbreviate Jan., Feb., Aug., Sept., Oct., Nov., and Dec. Spell out in running text when using alone or with the year alone. When a phrase contains a month and year alone, do not use commas. When a month, day, and year are included, separate the year with commas. Examples: School begins Sept. 5. He was born July 7, 1961, in Illinois. The demonstration began Thursday, Nov. 14. The grant ends in February 2014. Do not use the endings -st, -nd, -rd, or -th or any superscript on dates. Do not use of between a month and year: June 2012 (not June of 2012).

Times

For times, use figures in running text, except for noon or midnight: The course meets from 10:30 a.m. to noon. (However, in course catalogs, schedules, and tabular materials, 12 a.m. or12:00 a.m. may be used instead of midnight, and 12 p.m. or 12:00 p.m. may be used instead of noon.) Lowercase a.m. and p.m., separate from the figure with a space, and set off with periods:11 p.m., 12:45 p.m., 10 a.m. Avoid redundancies such as 10:30 a.m. Tuesday morning, 7 p.m. tonight, 12 noon, 12 midnight. In tabular material, be consistent when designating minutes, even if the time falls on the hour: 2:30–4:00 p.m. (Note the use of the en dash in the preceding example.) In other uses, minutes are not needed if the time falls on the hour: The reception will be held in the Neuberger Museum Study from 4:30 to 6 p.m.

Date and Time Ranges

When giving date or time ranges in running text, spell out the words from, to, and between: She attended college from 1986 to 1990. In other uses, use the en dash: the 2012–13 academic year. For the sake of parallel construction, the word to—never the en dash—should be used if the word from precedes the first element in such a pair: She attended college from 1998 to 2002 (not from 1998–2002). Similarly, the word and—never the en dash—should be used if the word between precedes the first element: They met between 10 a.m. and 5 p.m. (not between 10 a.m.–5 p.m.).

Decades

For decades, use either all four figures (the 1990s) with no apostrophe before the “s” or, if the meaning is clear in the context, the last two figures with an apostrophe to indicate the missing figures (the ’90s). When used in a compound form, use all four figures for clarity: the mid-1980s.

Centuries and Eras

For centuries, spell out first through ninth, use figures for 10th and above: photography’s first century; 20th-century art history. Do not use superscript on the word endings -nd, -rd, -st, and -th. 

Added 11/16/17: Era designations are usually expressed in one of two ways: either CE (“of the Common Era”) and BCE (“before the Common Era”), or AD (anno Domini, “in the year of the Lord”) and BC (“before Christ”). Use full capital letters with no periods. This is Chicago Manual style and an exception to current AP style, which uses A.D. and B.C. with periods.