Dictionary.com Word FAQs
Is 12 o'clock at night 12 a.m. or 12 p.m.?
If it is 12:00 a.m., then it is midnight. If it is 12:00 p.m., then it is noon. You can use either lower or upper case letters for A.M., a.m., P.M., p.m. and you can even leave out the punctuation. It is considered bad form to use these abbreviations without a specific number attached to them: We will do this in the a.m. We will do this tomorrow a.m. By the way, time period is redundant and period is almost always sufficient.