what was the first time that patient 42956 had the maximum amount of sodium until 04/2105?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 42956 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'sodium' ) and strftime('%y-%m',labevents.charttime) <= '2105-04' order by labevents.valuenum desc, labevents.charttime asc limit 1