when did patient 6215 during their last hospital encounter have the maximum asparate aminotransferase (ast) for the last time?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 6215 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 = 'asparate aminotransferase (ast)' ) order by labevents.valuenum desc, labevents.charttime desc limit 1