Setting due date for an activity +7 days after a custom date


I have been searching for an answer here and in the knowledge base, and it seems that it is not possible to do what I need, but I would love some confirmation:

I have a deal with a five-stage funnel and a custom date field called "end date". I want to create a new activity triggered by advancing the funnel from the 4th to the 5th stage. That's already working great.

The problem is that I want to set the due date of that activity to "end date + 7".

  • I cannot use the delay option and wait for seven days because "today + 7" is not necessarily the same as "end date + 7".
  • Nor can I use the due date field in the activity box because I can only choose "end date," but I need "end date + 7".

Am I doing something wrong? Thanks in advance.