Time-based conditionals for ROR

I'm running a ROR Database-driven website operating out of C9 IDE; It's a website where users submit text through an html form to collaborate on a story, one sentence at a time.

What I'm Trying to do: I need to figure out the syntax necessary to have the database purge itself every thirty days. Broadly, I need to create an if-then statement that can run independently of either user input or my input in the background, meaning that I need a script that can be in the background whenever the server is active. Input is appreciated!