when was the first time for a medication prescribed to patient 70950 since 56 months ago?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 70950 ) and datetime(prescriptions.startdate) >= datetime(current_time,'-56 month') order by prescriptions.startdate asc limit 1