$UseAmPm = 1; # 1 = use am/pm in times, 0 = use 24-hour times
The am/pm time system is strange. Don't be confused with for example history entries like
Revision 28 . . August 10, 2003 1:10 am by CliffordAdams [Restored and repaired] Revision 27 . . August 10, 2003 12:58 am by pD9E40230.dip.t-dialin.net
12:58am is 12 minutes earlier than 1:10am.
The easy explanation is that am starts at 12:00am (midnight), and pm starts at 12:00pm (noon/mid-day). It may be a little clearer if you look at an analog clock like this: [found through a Google image search]
The code includes the statement $hour = 12 if ($hour == 0); when using am/pm time. Am/pm time is a minor option which can be turned off to display 24-hour time. 24-hour time is uncommon in the United States--outside of the military almost everyone uses 12-hour time.