jbpm currently has no differentiation between created and started . A reassign can take place when the duedate ends or manually. The differences between the times can be calculated for create- and enddate with a hibernate query or whatever, not for the startdate (since it is not there)
using a custom extension to TaskInstance and some custom logging you can get the desired behavior. Patience and use of the source will guide you - we needed and got the same thing working, so I know it's possible.