Limitation: The data for first seen / last seen is tied to the AMP Setup -> Historical Data Retention value -> Inactive Client and VPN User Data setting. So you can only go as far back as you keep the data for.
Try this from the AMP CLI:
# dbc 'select mac, username, first_start as first_seen, last_stop as last_seen from client_first_last;'
If that's what you're looking for, you can throw it into a csv file using (header row would be cut out):
# dbcsv 'select mac, username, first_start as first_seen, last_stop as last_seen from client_first_last;' >> /tmp/client_first_last.csv
Extract /tmp/client_first_last.csv and open in your doc viewer to manipulate how you like.
The time output will be in epoch time, so you'd have to spin off another column to calculate human readable time for both first_seen and last_seen values. The formula to convert epoch time to normal human readable time is (epoch_time/86400)+25569.