what is the number of days since patient 17667 received the last magnesium sulfate prescription on this hospital encounter?
select 1 * ( strftime('%j',current_time) - strftime('%j',prescriptions.startdate) ) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 17667 and admissions.dischtime is null ) and prescriptions.drug = 'magnesium sulfate' order by prescriptions.startdate desc limit 1