Go to Audience > Contacts.
Search for the contact email address or mobile number.
Select the email address or mobile number to view the contact record.
This brings you to the Single customer view for the contact. The date of their most recent subscription is displayed in the top left of the Overview tab.
If a contact unsubscribes and then resubscribes, the date of the most recent resubscribe is shown.
Contacts subscribed prior to 15 October 2014 always display Unknown.