How to make task dates relate to an arbitrary start date?

Our project has a dynamic timeline that is all related back to one regulatory approval date. We don’t know when this regulatory approval will come through but we need all dates to relate back to it. When I enter tasks on my board, I want to say “10 days after” the approval date, for instance.

Is there currently a way to do this on I don’t know any of the real dates for my timeline but I want to view a “what if”. So enter an arbitrary approval date and all other dates will populate for a theoretical timeline we can easily view and adjust if our approval comes sooner or later than the arbitrary date.

I can accomplish this in excel with a simple formula but I would prefer to not keep a timeline outside of and have to recreate all those tasks there.