Assuming you want it to run a thread on device startup:
Create a second project and list it as an alternate entry point.
In your UiApplication or Application main(), check the argument passed to the project. Do your periodic stuff there via Thread.sleep and don't call enterEventDispatcher.
search for "autostart":
http://docs.blackberry.com/en/developers/deliverables/1076/development.pdf
Or if you want to do something once a user "starts" it, then look into creating a new thread to do your timing stuff. Override your screen's onClose() and use Application.getActivation().deactivate() to throw the screen into the background.
Or there's a other ways to do something like this like invokeLater, etc. Maybe eventlisteners may do what you need, but you didn't give a lot of details.