CREATE TABLE table_71680 ("Date" TEXT, "Tournament" TEXT, "Surface" TEXT, "Opponent in the final" TEXT, "Score" TEXT)

What is the date of the tournament where Kyoko Nagatsuka was the opponent in the final?
SELECT "Date" FROM table_71680 WHERE "Opponent in the final" = 'kyoko nagatsuka'
SELECT "Date" FROM "table_71680" WHERE "Opponent in the final" = 'kyoko nagatsuka'