when was patient 15821 prescribed for the last time a drug in 01/this year?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 15821 ) and datetime(prescriptions.startdate,'start of year') = datetime(current_time,'start of year','-0 year') and strftime('%m',prescriptions.startdate) = '01' order by prescriptions.startdate desc limit 1