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 BP." When designating eras with BC or AD, the abbreviation AD precedes the year and BC follows it: "Britain was invaded successfully in 55 BC and AD 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, 17th edition (2017), 9.34
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