Abbreviations

Q. When should you capitalize AM and PM?

A. Capitalize “AM” and “PM” not only in the morning and afternoon but at any time of day or night—unless you’re following Chicago style, in which case use lowercase and periods (10:30 a.m., 5:30 p.m.). See CMOS 10.41.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]