Delete All Past Unassigned Shifts as a rule set options

While working in the Supervisor Role, under the Shift management tab, we currently manage the backlog of our past unassigned shifts by selecting the "Delete All Past Unassigned".  This is simple enough, but it is manual.  Would like to have the option available in the rule sets to set an expiration time frame to automate this.  What I see now is this is cleared/deleted usually once a week.  I feel the option for both, manual and automated would satisfy any of your customers.