since 04/2105, when was patient 27172 prescribed for the first time for a drug?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 27172 ) and strftime('%y-%m',prescriptions.startdate) >= '2105-04' order by prescriptions.startdate asc limit 1