Hi Cori,
What data format is your duration estimate field? It needs to be a number format in order for the calculation to work. My calculation assumes that the duration estimate is a number of days.
If your users are typing the duration as text (i.e., "3 months"), Workfront wont be able to read this as a number to carry out any mathematical calculations.
If you can have your users enter the estimated duration consistently as a number, we could update the calculation to accommodate weeks or months. For example, the below code would subtract the number of months specified in the duration estimate from the due date.
ADDMONTHS({DE:Due Date},-{DE:Duration Estimate})
Best Regards,
Rich.