user: Question: Who presented their credentials at an Unknown date?
Columns: ['Name', 'Background', 'Title', 'Appointment', 'Presentation of Credentials']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['William H. Christensen', 'Non-career appointee', "Chargé d'Affaires ad interim", 'August 31, 1962', 'August 31, 1962']
SQL:

assistant: SELECT "Name" FROM "Table" WHERE "Presentation of Credentials" = 'Unknown';