The hook_cronapi($op, $function = NULL) allows you to define new cronjobs

### Example

    // Define custom cron functions
    function mymodule_cronapi($op, $function = NULL) {
      switch($op) {
        case 'list':
          return array(
            'mymodule_cronjob_1' => 'Cron-1 Handler',
            'mymodule_cronjob_2' => 'Cron-2 Handler',
            'mymodule_cronjob_3' => 'Cron-3 Handler',
          );

        case 'rule':
          switch($function) {
            case 'mymodule_cronjob_1': return '*/13 * * * *';
            case 'mymodule_cronjob_2': return '0 0 1 * *';
          }
          break;

        case 'execute':
          switch($function) {
            case 'mymodule_cronjob_2':
              mymodule_somefunction();
              break;
          }
          break;

        case 'settings':
          switch ($function) {
            // 'mymodule_cronjob_3' disabled by default
            case 'mymodule_cronjob_3': return array('enabled' => FALSE);
          }

        case 'configure':
          switch ($function) {
            case 'mymodule_cronjob_3': return 'admin/configure-modules-settings/xxx';
          }
      }
    }

    // Custom cron-function
    function mymodule_cronjob_1() {
      // Do some stuff ...
    }

    // Custom cron-function
    function mymodule_somefunction() {
      // Do some stuff ...
    }

    // Custom cron-function
    function mymodule_cronjob_3() {
      // Do some stuff ...
    }