when was for the last time until 20 months ago patient 27599 prescribed a medication via iv route?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 27599 ) and prescriptions.route = 'iv' and datetime(prescriptions.startdate) <= datetime(current_time,'-20 month') order by prescriptions.startdate desc limit 1