If iCal doesn't seem to acknowledge that you've changed your computer's date or time, quit it and open it again.
Symptom
After switching between a 24-hour and 12-hour clock in Date & Time preferences, iCal calendars may not reflect the change.
Some alarms may not work after changing the date or time in Date & Time preferences.
Solution
Quit iCal and open it again after making changes in the Date & Time System Preference pane.