Here's another tip on one of my favorite topics: Date Shortcuts.

Need to enter different start and due dates for tasks? Enter the date (you can use date shortcuts) in the due field then either select the start date from the drop down or enter just the day of the month, ie, 13. Outlook will convert the number to 1/13/2006 for you. You'll have different Due and Start dates and Outlook won't try to change them.

If you are having problems setting different dates (or want Outlook to calculate the date range), enter the start date first and backup to the due date field then type over the due date. If the due date is 7 days from the start date, type 7d in the due field and Outlook will calculate it for you. (Use Shift+Tab to reverse if you are using Tab to move around.)

New to date shortcuts? Learn more about them at:
http://www.outlook-tips.net/archives/2003/20030717.htm
http://www.outlook-tips.net/archives/2003/20030721.htm
http://www.slipstick.com/emo/2005/up050818.htm#naturaldate