So you want a formula that -- at the time you save the task -- can calculate the date that falls halfway between today's date and the due date?
This formula for use in a formula field takes half the difference between the date the task was created and its due date and adds that to the created date:
DateAdd("d",CInt(DateDiff("d",[Created],[Due Date])/2),[Created])
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers