Is your alarm clock set for 6am, 6AM, 6 A.M., or 6 a.m? (Don’t mind me; I’m just hitting the snooze button while you ponder that one.)
The answer here brings out the Latin dictionaries. This is an abbreviation, after all, just like “i.e.” or “e.g.” When it comes to time, “a.m.” stands for ante meridiem or “before noon,” and “p.m.” stands for post meridiem or “after noon.” So for those of you paying attention, you’ve figured out the answer.
The above time is correctly written as “6 a.m.” Notice the space between the number and the abbreviation? That’s a part of the correct answer too.
In an era of quick emails, writing the time as “10am” or “4pm” has been gaining popularity, but there are a lot of sticklers out there about this one. Give the numbers some space. Give the abbreviations the periods. Keep it in lower case, folks.