Fixed and rolling dates in Bookmarks and Dashboards

Make it so we can define whether the start date and end date should be a fixed date or a rolling date for when we come back to the Bookmarked Report or Dashboard.

Currently I have a set of bookmarks for the AB tests we are running on our site. I would like the start date of the report to be fixed and the end date of the report to be variable so I can look at the results of the test as it progresses.