This has been driving me nuts for years. Not sure, but it seems I have seen every variation listed below from different writers and sources when it comes to writing the time.

Can you guys tell me which one is correct? That is, if there is a “correct” way of writing it. I’m also wondering if it varies depending on country of origin. I do Fiction in the U.S.

7:45 AM
7:45 am
7:45 A.M.
7:45 a.m.

Thanks in advance for your help! Sorry if this has been asked before in this forum; If so, I missed that thread.