Please see: https://big.ideas.aha.io/ideas/APP-I-2605 That feature plus this request are really what we are after. The idea being that we have items that do not always take a specific amount of days. Product management would like the ability to change the release date and based on that have the schedule expand or compress based the percentage assigned to each milestone and phase. Sometimes we have to do releases based on an accelerated schedule as client needs shift and the release date is a hard target - they want the tool to automatically fill out the dates that we need to hit for all of the phases and milestones based on that concrete release date.
Thank you for your idea. We would recommend using release templates with predefined durations to create your typical schedule. You then have the flexibility to then make adjustments for releases which fall outside the norm.
Given the flexibility to adjust the dates and durations of release phases and milestones and other priorities, we are unlikely to implement this idea at this time. We hope you can understand.