The start date for a release is based on the start of the first release phase, milestone, or feature start date. If no phases, milestones, or features are added to a release, the release start date used for reporting will default to the Development started on value found in the release details.
The Development started on value also serves as the start date for the burn down chart found in the Releases > Details > Progress tab view regardless of whether a release contains phases/milestones.
Note: if you send a release from Aha! to a development tool through an integration, the Release start date will be determined by the field mapping you establish for that integration. For example, if you map the Development started on Aha! field to the Start date Jira field, then your release's start date in Jira will be determined by the day that the first feature is moved out of Not started status.