On the 5D it's...
1. Press the Menu button
2. Select 'Date/Time' from the menu.
3. Highlight the day / month / year / minute / hour element you want to change
4. Press 'Set' on the selected item.
5. Scroll using the back wheel to the time / day etc.
6. Press 'Set' to return to step 3. and select another element to change.
7. When you are finished, highlight 'OK' to save changes.