The WS-HumanTask specification does not define a time for when a Task is completed or released as part of the TaskData, so this is not included by default in the Task.
It would be possible to register a listener that would record this information whenever a task is completed / released. The time a task is completed is probably also available in the history log as the time the human task node instance that requested it was left.
It would be possible to extend TaskData with (an) additional field(s) to store this information (that would be populated whenever the task is completed / released), if you would like to record this kind of information as part of the task data.
Kris