Look in the Application Suites section and you may find in the properties of an application the installed date. Seems to vary for me by application.
Otherwise, you could look at the file properties in the inventory and see the date discovered. This becomes more accurate if it is installed after LANDesk was first installed as it can't see when it first appeared on a machine if LANDesk wasnt there then. You need to be certain you are getting a software scan each day though