when did patient 73713 receive a prescription last month for the first time?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 73713 ) and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-1 month') order by prescriptions.startdate asc limit 1