Conversion of numbers to dates

This query is meant to retrieve a count of the no. of stocks that have prices in the last 8 days.

select E.exchangename,PH.marketdate,
            count(*) as PriceToday
            from exchange as E left outer join pricehistory as PH
            on E.dsexchangecode=PH.dsexchangecode
            where PH.marketdate > (select CONVERT(nvarchar(30), GETDATE(), 112)-8)
            group by E.exchangename,PH.marketdate


The problem is that the dates are stored in format yyyymmdd, so it's doing arithmetic calcualations not date calculations.  How do I convert it to dates?