how change a boolean 30 days late of update_at in rails

how change a boolean 30 days late of update_at in rails

i have a list of records in Rails, and this records have a boolean field
to make it public or private. I what to change to private every public
record with update_at >= 30 days ago. And every record get the 30 days,
change automatic to boolean private.
But i don't know when to start. Advices, links and tips are welcome. Thank
you!