a.m. / p.m.: style
Capitalization, punctuation, and spacing of the abbreviations for times before and after noon.
Write a.m. and p.m. in lowercase letters with periods and a space after the time: 9 a.m., 6:30 p.m. Use figures, not words, for the hour. Prefer noon and midnight to 12 p.m. and 12 a.m., which are easy to mix up. Do not add in the morning or in the evening after a.m. or p.m. Do not write 9:00 a.m. when the time is on the hour; 9 a.m. is enough.
Examples
Use The hearing starts at 9 a.m.
Use Dinner is at 7:30 p.m.
Avoid The hearing starts at 9 AM.
Avoid Dinner is at 7:30 pm.
Small capitals (9 A.M.) appear in some book typography. All-cap AM and PM without periods are common on clocks and in informal digital text. Neither is the default in running prose. A sentence that ends with a.m. or p.m. takes only one period.