when did patient 24971 receive for the last time a foot culture microbiology test when they came to the hospital first time?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 24971 and admissions.dischtime is not null order by admissions.admittime asc limit 1 ) and microbiologyevents.spec_type_desc = 'foot culture' order by microbiologyevents.charttime desc limit 1