years—Use figures, without commas, for years with four or fewer digits: 1948, 1776. Use figures, with commas, for years with five or more digits: "Radiocarbon dating indicates that the campsite was in use by about 13,500 B.P." When designating eras with B.C. or A.D., the abbreviation A.D. precedes the year, and B.C. follows it: "Britain was invaded successfully in 55 B.C. and A.D. 1066." (31)
Use s (no apostrophe) to indicate spans of decades or centuries: the 1890s, the 1900s. (See plural figures and letters.)
Set off a year with commas when it follows a month and date: The Senate approved the recommendation at its April 13, 2007, meeting.
Do not use a comma to separate a month and a year: June 1988.
Years are the sole exception to the rule of spelling out numbers that begin sentences. (See numbers, figures or words? )
(31) Chicago Manual of Style, 14th edition (1993), 8.41
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