when did patient 94229 receive potassium chloride prescription first this month?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 94229 ) and prescriptions.drug = 'potassium chloride' and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-0 month') order by prescriptions.startdate asc limit 1