fixed date range that "moves" with project date range

Today, it is September 14, 2017.

If I look at "last month" today, "last month" will give me August 2017. However, if I change my Workspace project to April 2017, I would expect "last month" to show March 2017, but it remains as August 2017 since it is currently September 2017.

It would be awesome if this custom date range "moved" depending on the date range being used in Workspace.