when was the first efavirenz prescription given to patient 13536 when they came to the hospital last time.
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 13536 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and prescriptions.drug = 'efavirenz' order by prescriptions.startdate asc limit 1