Have you ever tried to make a filter? You do so via SQL, but the good news is that for this, the SQL is already there for you. If you go to Manage | Filters, you can create new filters there. If you go to Reports | All reports, and then Reports\Discovery and Inventory\Inventory\Windows\Inventory Agent\Inventory Capture. If you clone and edit this report it shows you the SQL you need.
That report is a bit of overkill, but essentially you're going to select guids from VComputer with a join on vResourceUpdateSummary with something like this:
HAVING DATEDIFF(day,max(rs.ModifiedDate),getdate()) >= 15
If you talk to anyone that knows SQL, they'll understand. Your filter will probably look something like this:
select vc.guid
from vcomputer vc
join vResourceUpdateSummary vrs on vrs.ResourceGuid = vc.guid
HAVING DATEDIFF(day,max(vrs.ModifiedDate),getdate()) >= 15
I haven't tested this, but it's moderately close. It might need a groupby clause.
Let us know how far you can get with either method, and what you still need. Before you go making a filter though, see if that report even shows you the systems you need.
OH! And you can export the list of systems in that report and simply push the agent to them... That also works.