Is it possible to set a search or filter for records with a date that is NOT in the last N months? 

I am trying to set it up so that our admin can easily see which records have NOT had an assessment done in the last 6 months. I see that there is an option in the drop-down that says "is not" - but I get the impression that is only for a specific date and not for a date range.  

Note: assessments are in a sub-form.  I am viewing a report for records of the main form with a column for the assessment sub-form that shows the date the assessment was completed. There may be multiple records for assessments for any given main record - I need to sort and show which main forms have no assessments associated with them over the last 6 months.

Thanks!
Wanda