Need expression for opening stock in inventory

[Anonymous]

Hi all,

i need expression for finding opening for stock calculation

now i have closing stock calculation

=sum({<CYYear=,CYMonth=,Period=,PostingDate={"<=$(#vInvDate1)"}>}Quantity) 

vInvDate1=  =date(max([Posting Date]),'DD/MM/YYYY')

kindly help to change the same expression into  opening stock calculation..

Advance thanks

Sirajahamed S