during this hospital visit how many days have passed since the first time patient 6170 was prescribed carbamazepine?
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 = 6170 and admissions.dischtime is null ) and prescriptions.drug = 'carbamazepine' order by prescriptions.startdate asc limit 1