in the previous month, when was the first time patient 19175 was prescribed thiamine hcl?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 19175 ) and prescriptions.drug = 'thiamine hcl' and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-1 month') order by prescriptions.startdate asc limit 1