CREATE TABLE table_name_23 (date VARCHAR, surface VARCHAR, score VARCHAR)
SELECT date FROM table_name_23 WHERE surface = "clay" AND score = "6–1, 6–4"
ดินเหนียวผิวคือวันใดและคะแนน 6–1, 6–4
What day was the surface clay and the score 6–1, 6–4?