How to define [today's date minus x days] in a smart list?

Hi,

I'm trying to create a program whereby if a person hasn't opened or clicked on emails in the last three months they're paused for a time to avoid emailing dormant contacts.

I want to ensure new leads that have entered the system recently are NOT caught by this process.

Is there a way to in the smart list to say:

Person was created

On or before

{{system.Date}} - 90 days

So any new people added to Marketo in the last 90 days are excluded from the program?

Thanks.