To have Entourage use 24-hour time, open System Preferences, and then open
International Preferences. Click the "Formats" tab. Under "Times", click
the "Customize" button. The first number that appears is probably the hour.
Click on it, and then select "00-23". Then, click at the end of the text
field, and press Delete to remove the AM/PM indicator. Click OK and then
quit System Preferences. When you relaunch Entourage, you should see that
it is now using 24-hour time.