Need to detect the time when the activity added after every action made to the activity.

Summary:

Hi,

when we add an activity it's added in a certain time let's say 02:00 PM then we assign it to a resource at 02:15 PM for example, now the system only captures the assigned time which is 02:15 PM to be the start time of the activity. I need to detect the actual time when this activity added, how can I find it ?