See #api documentation > new feature level: 475:

• GET /events: realm_user events with op: "update" are now sent when the date_joined field is updated after an imported stub user or a user created via the API logs in for the first time.
• GET /users, GET /users/{user_id}, GET /users/{email}, GET /users/me: The date_joined field is initially set to the account creation time and is updated to the time of first login for imported stub users and users created via the API.

Tim elaborates that this means two changes for the mobile app:

• Showing something to the effect of "Imported on" rather than "Date joined" as the string if is_imported_stub is true for a user whose card information is being displayed.
• Being able to handle events that live-update is_imported_stub and date_joined if/when such a user joins in a way that converts the account from an imported stub into a real account.

and offers a tip for manually testing those changes (and producing screenshots):

There's a new "Imported user" account generated by populate_db that should make it easy to test with a Zulip development environment.

References

• #api documentation > new feature level: 475
• Add "Imported users" tab and indicate imported stub user in popover. zulip#37587