instruction: in the previous month, when was the first time patient 19175 was prescribed thiamine hcl?
response: 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 LIMIT 1