when was patient 53015 prescribed nystatin ointment first until 08/2103?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 53015 ) and prescriptions.drug = 'nystatin ointment' and strftime('%y-%m',prescriptions.startdate) <= '2103-08' order by prescriptions.startdate asc limit 1