How to filter report to exclude sub-form dates

How to filter report to exclude sub-form dates

I'm trying to create a list that identifies accounts that have no activity in the last 4 weeks.

My main Accounts form has a sub-form for Activities, so what I was hoping would work is:

show  all  rows  from  Accounts [!Activities.Activity_Date in last 4 weeks]

but the use of ! to create a negative doesn't work.

If I try to use >, it won't filter Accounts that have activity both before and after the 4 weeks.

Is there a !> function, or some other means to say "not after"

I've tried using other deluge statements such as notContains, but can't seem to find anything that works.

Any suggestions?