when did patient 9294 last have minimum magnesium on their first hospital visit?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 9294 and admissions.dischtime is not null order by admissions.admittime asc limit 1 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'magnesium' ) order by labevents.valuenum asc, labevents.charttime desc limit 1