when did patient 26793 receive last a prescription for zofran since 147 months ago?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 26793 ) and prescriptions.drug = 'zofran' and datetime(prescriptions.startdate) >= datetime(current_time,'-147 month') order by prescriptions.startdate desc limit 1