CALC FUNCTION ON TABLE Purchase Line

HI
AM CORONTLY GO TO CREAT A REPORT BASED ON TABLE Purchase Line IN NAV CLASSIC CLIENT (2009 R2) THE OBJECTIV OF REPORT IS TO CALC DIFFERENC BETWEEN TWO POSTINGE DATE
PLEAS HELP ME IF ADD WITH C/AL GIVE ME THIS LINE IF NOT OR WITH ANOTHER IDEA TRANSFERT TO ME
THANK YOU