3 of the frequency constants used in class.CronJobTrait.php are not defined in
class.CronJobInterface.php
RECUR_2H
RECUR_3H
RECUR_12H

PHP Fatal error:  Uncaught Error: Undefined class constant 'RECUR_2H' in
/home/blah/public_html/lib/classes/Async/class.CronJobTrait.php:93

To create this error I called $this->frequency = self::RECUR_WEEKLY;

To fix it, in class.CronJobInterface.php - although haven't tested those
frequencies ... but will now...
    const RECUR_2H = '_120m';
    const RECUR_3H = '_180m';
    const RECUR_12H = '_720m';