Give the dates of creation for documents that have both budget type codes 'GV' and 'SF'.
select t1.document_date from documents as t1 join documents_with_expenses as t2 on t1.document_id = t2.document_id where t2.budget_type_code = 'GV' intersect select t1.document_date from documents as t1 join documents_with_expenses as t2 on t1.document_id = t2.document_id where t2.budget_type_code = 'SF'