Invoice Prepared before 30 days only by Item description

SELECT 
a.CardCode as 'Code',
a.Cardname as 'Customer Name' ,
a.Docentry ,
a.Docnum as 'Inv.No' ,
a.Docdate as 'Inv.dt' ,
b.U_part_no as 'PartNo.', 
b.Dscription,
b.U_inv_f_text as 'Inv.Txt',
b.Price as 'Unit SP',
b.Quantity,
b.linetotal as 'Total SP'  ,
 Case
when a.DocType = 'I' then 'INVOICE'
when a.DocType = 'S' then 'INVOICE'
End 'Doctype'
from OINV a inner join INV1 b on a.docentry = b.docentry
where b.Dscription like '%Rental Charges%'
and  (a.docdate =  CONVERT(VARCHAR(10),DATEADD(MONTH,-1,GETDATE()+1),120) )