The Flight Tracker supports flights from 3 days in the past to less than 3 days in the future. Currently there is not another feature that exposes flights farther out than 3 days. If you are interested in support for this feature, please submit a request to the FlightStats Help Desk. Your feedback helps us prioritize future work.
While you can not use the Flight Tracker for flights more than 3 days in the future, you can sign up for a flight alert on a flight many months in the future. This is a premium feature that requires subscription.