Well, for future reference, please TellTheDetails (←click), because that makes a huge difference.
From what I can see, the CronExpression class requires 6 fields (standard Unix cron only has 5), and the ? in the final one (which I took to be part of your question) means 'no specific value', which I suspect equates to the same thing as *; but not having used the system, I can't be sure.
My advice: suck it and see.
subject: Cron expression : run every midnight 23:59:59