instruction: last month, when did patient 21110 receive the first prescribed prescription?
response: SELECT prescriptions.startdate FROM prescriptions WHERE prescriptions.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 21110) AND DATETIME(prescriptions.startdate, 'start of month') = DATETIME(CURRENT_TIME(), 'start of month', '-1 month') ORDER BY prescriptions.startdate LIMIT 1