Fuzzyclock is a great clock for Mac OSX. This page has some information on configuring it.
Open System Preferences
Click on Accounts
Click on your account
Click on the Login Items tab at the top
Click on the + icon and browse to FuzzyClock to add it
Now Fuzzyclock should appear by default on startup
Open System Preferences
Click on Date and Time
Uncheck the “Show date and time in menu bar” checkbox
Now the default clock should disappear from the menu bar