Q. I’m pretty certain CMOS said to omit the “of” in month-year references (“he graduated in May 1999,” not “he graduated in May of 1999”), but I can’t for the life of me find this in the 17th edition. Is there a reason it is no longer covered? And do you have guidance?
A. We haven’t been able to find such a rule in earlier editions of CMOS, but according to Bryan Garner (author of CMOS chapter 5), it’s best to leave out “of”: “February 2010 is better than February of 2010”; see Garner’s Modern English Usage, 5th ed. (Oxford, 2022), under “Dates. B. Month and Year.”
That advice is presumably directed at writers and editors. In speech, adding an “of” between month and year is relatively common—and there’s nothing inherently wrong with doing so. Similarly, Chicago style is to write “July 5,” whereas people typically say “July 5th.” Rules intended for writing, which tend to favor precision, don’t always translate to speech.
[Editor’s update: It turns out that Garner’s advice is in the Manual after all. See the entry for of in the usage glossary at CMOS 5.250: “Avoid using this word needlessly after all, off, inside, and outside. Also, prefer June 2015 over June of 2015. To improve your style, try removing every of-phrase that you reasonably can.” Our focus on CMOS’s coverage of dates led us astray. Thank you to a reader for kindly bringing this to our attention.]