How do I calculate default date based on value of another date?

I have a date field "EndDate" that get populated from a lookup on another field. I want a second date field "Extended End Date" to default based on the value of "EndDate". Example: When user selects value from lookup end date is set as 01/31/2011, extended end date should immediately default to End Date + 6 months. Now if the user changes "End Date" then "Extended End Date" should adjust accordingly. I would also like for the user the override the "Extended End Date" with their own value before saving.
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