For the patient with the highest lactate dehydrogenase in the normal range, when was his or her data first recorded?
highest lactate dehydrogenase in the normal range refers to MAX(LDH < 500); when the data first recorded refers to MIN(First Date);
SELECT T1.`First Date` FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID WHERE T2.LDH < 500 ORDER BY T2.LDH DESC LIMIT 1