Q. CMOS 9.37 seems pretty straightforward: “Times of day in even, half, and quarter hours are usually spelled out in text.” I’m an editor on contract with one of the larger self-publishing companies. I recently got this note from an editorial staffer: “In several instances, you changed references like 1:00 a.m. and 10:00 p.m. to one a.m. and ten p.m. . . . If you have found specific Chicago rules to support the changes you’ve made, please let me know and I’ll be happy to pass the manuscript through as is. However, I don’t know of a rule that would allow that. If you don’t know of one either, would you please change the time references with a.m. and p.m. back to numeral form and resend?” For seven years, I’ve been spelling out times of day ending with :00, :15, :30, and :45, with or without a.m./p.m., if they did not seem particularly significant in context. This is the first time it has been called into question.
A. Your correspondent is correct; expressions like “one a.m.” are not Chicago style. Better to write “1:00 a.m.” or “one in the morning,” depending on whether you mean the time exactly or not, respectively.