Revision Deletion: (Relative) Revision Age

Currently the Revision Age is absolute, from "today".

It would be useful if it could optionally be set to relative from the current revision date. For example, I like to have some revisions, no matter how old the current revision. So I'd like to keep the current revision, and all revisions that are no older than 6 months of the current revision. If my current revision is already 6 months old, I'd like to keep all revisions that are between 6-12months old.