Selecting a week back date range

select
ship_id,
item_no,
sum(item_qty) as item_qty,
sum(amount) as amount,
avg(item_price)as item_price
into #scanCt
from openquery(PP_NEW,
'select item_no,item_qty, amount,item_price,category,
invoice.ship_id,invoice.bill_id,invoice.ship_dt
from invoice left outer join line_it
on invoice.invoice_no=line_it.invoice_no
left outer join ship_to
on invoice.ship_id=ship_to.ship_id
where status=[Posted]
and type=[Invoice]
and invoice.ship_dt>={01/01/12}
and item_qty>0
and lscanpo=.T.
and invoice.bill_id =[CHOMEV]
and ship_to.ship_id in ([[HD04279],[HD04280],[HD04282],[HD04283],[HD04284],[HD04285],[HD04286],
[HD04287],[HD02401],[HD02403],[HD02404],[HD02405], [HD02406],[HD02407],[HD02408],[HD02409],[HD02410],
[HD02412],[HD02414],[HD03401],[HD03403],[HD03480],[HD03481],[HD03482], [HD03484],[HD03485],[HD03486],
[HD03488],[HD04551],[HD03402],[HD03406],[HD03407],[HD03408],[HD04501],[HD04502],[HD08539])
')

where bill_id=@customer and
ship_dt between @startDate and @endDate
group by ship_id,item_no

users are entring a start date and end date 02/01/12 to 04/12/12
I am loking for to be able to calculate dates a week mack Mon to Sunday

So if the end date is falling in the week of (04/09 – 04/14) the week back dates will be 04/02/12 to 04/08/12