How to Determine the Date when a task was assigned and when it was completed

Hi
I am trying to determine the dates when a task arrived to the group. When the task was assigned to a user and when the task was completed. This would help us in understanding how much time was the task lying open before someone worked on it.
I am using an Rest API to fetch that Data from the portal . /bonita/API/bpm/archivedHumanTask?p=0&c=100&f=caseId={{caseId}}&o=archivedDate%20ASC&o=displayName%20DESC

Here there are 3 seperate dates.
1) Assigned Date
2)Reached State Date
3) Archived Date

I do understand that Archived date will be the time when the task was completed. But My concern is to find out when the task was assigned to group and lying open. The date when a user picked up the task .

Please let me know if you have any other way of getting the same information.

Regards
Dibyajit Roy