I was getting tired setting this every time I started and yes I am a worthless n00b.
Source:
http://simpeforum.ambertation.de/sims2/Force-12-Hour-Clock-FixThere is a new UserProp setting called "force12HrTime", unfortuneately EA somehow programmed it so that it's an "incompatible" setting with the current property revision, so anytime it's added to UserProps.xml, it is removed.
However, if you go into your "Documents\EA Games\The Sims 2\Config" folder, and create a file named "userStartup.cheat" (or edit it if it already exists) and add the following line:
boolProp force12HrTime true
That will make it so everytime Sims 2 is loaded, the 12 hour clock will be set. AFAIK this setting only works with Bon Voyage expansion pack.