Date Parameters for dynamic PIVOT where clause

Hi,
I have  following query where I need to pass Dates parameters to the WHERE clause.
If I input the dates to the WHERE clause as Strings, then the records don't get filtered based on the dates. If I input the dates as DateTime, I get error saying 'Conversion failed when converting datetime from character string'.

Any suggestions?
Thanks in advance:

DECLARE @StartDate as DateTime,@EndDate as DateTime,@StartDateStr as varchar,@EndDateStr as varchar,@cols NVARCHAR(2000),@PIVOTcols NVARCHAR(2000),
@SQL nvarchar(max),@ProductID as varchar(max)

SET @productID = 9999
SET @StartDate = '04/11/2009'
SET @EndDate = '04/14/2009'
SET @StartDateStr = '04/11/2009'
SET @EndDatestr = '04/14/2009'


SELECT  @cols = 'A,B,C'

SELECT  @PIVOTcols = 'A,B,C'

SET  @SQL =

'Select OrderID,ProductId,OrderDate,ShipDate, '  + @cols +
' FROM(
Select  
OP.OrderID
,OP.ProductID
,convert(varchar,O.OrderDate,101) as OrderDate
,convert(varchar,O.ShipDate,101) as ShipDate
,odt.ST
,odt.TA
from OP
inner join O on OP.OrderId = O.ID
Inner join od on O.ID = od.orderid
Inner join odt on odt.OrderDetailID = od.ID
'AND convert(varchar,o.ShipDate,101) >= convert(varchar,' + @StartDate + ',101)
AND convert(varchar,o.ShipDate,101) <= convert(varchar,' + @EndDate +',101) PIVOT
(max(TA) for ST in (' + @PIVOTcols + ')) as PivotTable '

PRINT (@SQL)