Report Usage (show those that are not used)

How do I identify which reports haven't been accessed in the last 12 months?

Below is the script to show the current report usage... but i need to know which ones haven't been accessed

select        c.name,
              e.userName,
              e.format,
              e.parameters,
              e.timeStart,
              e.status
from          REPORTSERVER.DBO.EXECUTIONLOG  e
              left outer join charon.reportserver.dbo.catalog c on e.reportID = c.itemID
where         
       e.timeStart >= dateAdd(mm,-6,getDate())
order by timestart desc