And even after ten years, you'll get an answer to your question.
The display format of the clock is dependent on the locale settings. In /etc/sysconfig/language you can change to whatever locale you like (and is installed).
If you however refer to the format of the "date" command in bash, changing the locale setting will not switch to the am/pm setting (at least not on my system). Here, I solved the problem by setting an alias in .bashrc like
alias date="date +%r"
There are many other formatting options available. Have a look at the manpage.