There are many ways to create date-driven views in Aha! and the order of the objects on those views is based on either the start or due date.
In the Releases -> Overview and Reports -> Roadmaps -> Releases screens, releases are ordered by what starts first since that is a planning screen.
In the Reports -> Roadmaps -> Custom views, the order is based on the date an object is due since reporting on timeline is about sharing what's being delivered, when.
The Reports -> Roadmaps -> Features view orders releases based on the internal or external release dates, depending on which you have chosen to display.