Leads created 4 weeks ago on a rolling window

How do you setup a smart list to look for leads created exactly 30 days ago (or 4 weeks)

The smart list above is then used into a batch smart campaign that runs every day and looks at the leads created 4 weeks ago on a rolling window. So each day that passes we look at 30 days in the past and see leads that have been created and send them an email.