Collection based on software not installed in last 28 days

Hi All,

Would somebody be able to help me with a query I am trying to write to populate a collection in SCCM?

I am trying to find all systems that have an application installed but have not had that application installed in the last 28 days.

I am able to create the query to find all systems that have the software installed, and can then add the paramater of installed since a specific date, but I would like to use the datedif() function rather than a static date so the collection is always updated on a schedule.

I'm afraid it is one step too far for my current ability with queries!!!

Thank you