A few months ago, we agreed that apostrophes do not belong in plurals: To make a plural, generally you add an s or es. No apostrophe. The same rule applies when you are referring to a decade, say, the 1920s. It is absolutely fine to put a letter after a number without an apostrophe between.
If, however, you decide to drop the 19 from 1920s, you would insert an apostrophe to show that something is missing: the ’20s. (After all, that is one of the apostrophe’s jobs: to show that something has been removed.)
You could also spell out the abbreviated form as “the twenties,” keeping it lower case unless you are talking about the Roaring Twenties, a distinct historic era. The Chicago Manual of Style, NEHGS’s style guide of choice, says, “Decades are either spelled out (as long as the century is clear) and lowercased or expressed in numerals.” I’ll repeat: As long as the century is clear. In genealogical writing, we’re often discussing such long spans of time, and precision is so essential, that we will probably always want to use the form 1920s, to distinguish the twenties decade from the 1820s or the 1720s or the 1620s—and soon from the 2020s.
Here are some other basic guidelines for writing dates:
When discussing the century, spell the ordinal number:
seventeenth century; twenty-first century
When using the century as an adjective preceding a noun, use a hyphen between the number and the word century:
seventeenth-century artifacts; eighteenth-century migration patterns; twenty-first-century fashions
In genealogical writing, it’s common to use the day-month-year style, which requires no comma:
10 April 1909
If you are writing narrative text and choose to use the month-day-year style, place a comma between the day and the year—and also after the year if it does not end the sentence:
My father was born April 10, 1909, at the family home in Bellaire, Ohio.
If you are giving only the month and the year, or a holiday and a year, you do not need a comma:
April 1909; Thanksgiving 2014
Regardless of how specific dates are pronounced in speech, use cardinal numbers when writing dates:
April 10, 2015 (not April 10th)
Exception: It is fine to use ordinal numbers in Quaker dates:
3 1st month 1653
Range of years and double dating
When expressing a range of years with a dash between the years, pick one of two styles: the full-number style, giving each numeral, or the abbreviated style. Be consistent!
Full-number style: 1894–1895; 1900–1901
Abbreviated style: 1894–95 but 1900–1; 1899–1901
When expressing double dates (necessitated by the switch from the Julian to the Gregorian calendar; see “Double-dating”), use a slash between the years, and use either the final digit of the second year or the final two digits. Again, be consistent!
1715/16; 1722/23 but 1700/1 (not 1700/01)
Do you have questions about how to write dates? Would you like to see posts on similar topics? Add a comment with your questions or suggestions, and I’ll try to address them in future posts.
The Chicago Manual of Style, 16th ed. (Chicago: University of Chicago Press, 2010), 476.