what was the last time that patient 96774 had a maximum value of potassium, whole blood until 05/2105?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 96774 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'potassium, whole blood' ) and strftime('%y-%m',labevents.charttime) <= '2105-05' order by labevents.valuenum desc, labevents.charttime desc limit 1