You can use the Python API to loop through all users within your organization and then all of the content for each user, then find the created and modified properties of each item. They're stored in epoch time so you'll just need to convert the time that the influx started to epoch time to make the comparison.
The "clone" sample has some of what you need:
clone_portal_users_groups_and_content | ArcGIS for Developers
It generates a list of all users, and then loops through each users content. Instead of moving things or generating a mapping, you just need to get the created date:
for item in user_content:
print("epoch time of created timestamp: {}".format(item['created']))
would give you the epoch. You can take this a step further to give you a formatted string:
for item in user_content:
print("item created: {}".format(time.strftime('%Y-%m-%d %H:%M:%S', time.localtime(item['created']/1000))))
