Simplify Setting Due Dates in Workflow

As a SuperPlänner, I want to set due dates depending on start dates, so that I don't need to enter fix dates. That would avoid potential errors and would safe time when defining the process.

This idea was raised by @Marving Ludger

"We see we can only position “fix” dates on the workflow in the screenshot, but we would like to put “flexible” dates : Day +2 for example.