when did patient 96833 receive a prescription for the first time during this month, via pr route?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 96833 ) and prescriptions.route = 'pr' and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-0 month') order by prescriptions.startdate asc limit 1