A dynamic email trigger based on a past date

Need help setting up a campaign/program where I can send an email to someone six months after a specific date. The date is stored as a field in his/her custom object.

For example, I want the system to evaluate everyone's CDO's and on the day, everyone whose associated date (within the CDO) is exactly 6 months ago, gets sent an email.

I have been able to set this up for new customers. If a new CDO gets added in today, they get added into the campaign  where they will wait 6 months before receiving the email.

I need a solution for everyone in the last 6 months, so that they dont miss out.