CREATE TABLE table_name_86 (date VARCHAR(255), time VARCHAR(255), meet VARCHAR(255), nationality VARCHAR(255))
What is Date, when Meet is "World Championships", when Nationality is "United States", and when Time is "1:46.68"?
    
SQL: SELECT date FROM table_name_86 WHERE meet = "world championships" AND nationality = "united states" AND time = "1:46.68"