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 > Gantt and Roadmaps > Strategy views, releases are ordered by the Release Date (internal) field. This is specifically tied to the diamond marker that appears for each release within these views.
In the 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 Roadmaps > Features view orders releases based on the internal or external release dates, depending on which you have chosen to display.