what time was patient 1902 prescribed for a medication for the last time since 05/2105?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 1902 ) and strftime('%y-%m',prescriptions.startdate) >= '2105-05' order by prescriptions.startdate desc limit 1