You often want to be able to just write some PHP against your magento system to export, or update bits of data. In the module config file, under the etc folder you need to add a crontab element as a child of the main config element.

<crontab>
  <jobs>
    <stockupdate_updater>
      <schedule><cron_expr>*/30 * * * *</cron_expr></schedule>
      <run><model>stockupdate/updater::import_stock</model></run>
    </stockupdate_updater>
  </jobs>
</crontab>