Date (Min/Max) in report builder

I am trying to build a report to see all our customer who haven't created a campaign in more than 2 weeks. We have a field in the MDA for number of campaigns created for each day. I created a report that shows the MAX date for when a campaign was created and then want to add a fitler where MAX date is less than 14 days ago. when I select the Max date field in the filter, it is asking me to put in a text value. What's the best way to accomplish this?