on their last hospital visit, when was the last time patient 80120 had the maximum po2?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 80120 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'po2' ) order by labevents.valuenum desc, labevents.charttime desc limit 1