This is for a new install of Portfolio. The documentation says that sprint start dates are based on the start date for releases. I have 3/16/15 set for all of my releases but my sprints are starting on 3/23 which is one week off for my 2 week schedule.
Everything in the backlog is set to the same date (3/16).
Removing Earliest Start dates from all of the backlog items and letting it calculate still has sprints starting on 3/23.