SELECT date_to FROM Project_Staff ORDER BY date_to DESC LIMIT 1
What is the last date that a staff member left a project?
[Schema (values) (types)]: | tracking_grants_for_research |  Document_Types : document_type_code (text) , document_description (text) | Documents : document_id (text) , document_type_code (text) , grant_id (text) , sent_date (number) , response_received_date (text) , other_details (number) | Grants : grant_id (text) , organisation_id (text) , grant_amount (text) , grant_start_date (number) , grant_end_date (text) , other_details (number) | Organisation_Types : organisation_type (text) , organisation_type_description (text) | Organisations : organisation_id (text) , organisation_type (text) , organisation_details (text) | Project_Outcomes : project_id (text) , outcome_code (text) , outcome_details (text) | Project_Staff : staff_id (text) , project_id (text) , role_code (text) , date_from (number) , date_to (text) , other_details (number) | Projects : project_id (text) , organisation_id (text) , project_details (text) | Research_Outcomes : outcome_code (text) , outcome_description (text) | Research_Staff : staff_id (text) , employer_organisation_id (text) , staff_details (text) | Staff_Roles : role_code (text) , role_description (text) | Tasks : task_id (text) , project_id (text) , task_details (text) , eg agree objectives (number);