When users are added to your database, this is tracked in their Role Membership History in Core. The easiest way to tell the correct date is to locate the All School role, as this is the role automatically given to every user in the system upon their record being created.

As a Platform Manager:
  1. Navigate to Core.
  2. Select Users > Edit User Profile Data.
  3. Search/select the user.
  4. Scroll to the lower left under System Information.
  5. Click Role Membership.
  6. Click the View History link on the upper right (below Edit).
  7. Locate the All School role, and view the Created date.