Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Use BCE, CE, a.m., p.m., No., and $ only with specific dates, times, numbers, and amounts
Abbreviations
Mechanics

The abbreviations BCE (before the common era) and CE (common era) both follow a date. Common alternatives are BC, which follows a date, and AD, which precedes a date.

Avoid using a.m., p.m., No., or $ when not accompanied by a specific numeral: in the morning (not in the a.m.).