Time and dates - Time - Abbreviations - Part III. Style 20 spelling

A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Time and dates - Time
Abbreviations
Part III. Style 20 spelling

You may use the abbreviations a.m. (ante meridiem, or before noon) and p.m. (post meridiem, or after noon) in text and elsewhere to designate specific times. The abbreviations should be lowercase, in roman type, with no intervening space. Do not combine them with in the morning, in the evening, or o'clock; see also 23.1.5.