I am in process of migrating our projects from manual way of planning timelines to automatic way provided by JIRA/Portolio.
However, after creating plan on portfolio, the start date of plan is when it is created on portfolio. I am not able to update start date, when project was actually started.
I understand that portfolio automatically calculates start/end dates of features based on resource strength, working days and estimates etc. And actually, I also want it to be calculated automatically.

However, I am not able to update first start date of plan. The project was started 1 month ago, and portfolio is shows it as started on 16 Aug, hence end date is also delayed (1 month later, than expected).
How we can update start date of project?