when was patient 728 given a alkaline phosphatase lab test for the last time?
select labevents.charttime from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'alkaline phosphatase' ) and labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 728 ) order by labevents.charttime desc limit 1