I got a strange value for only a couple of rows in my sysjobhistory table for the run_duration column.
Value: -954441829
The job kicked off at 1:54am on 11/1/15. This is the day we changed from daylight to standard time.
So the job thinks it ended before it began, hence the negative duration. Problem solved! Had me perplexed for a few days until I checked the start date and time.