when during their last hospital visit was patient 4718's first test for atypical lymphocytes?
select labevents.charttime from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'atypical lymphocytes' ) and labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 4718 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) order by labevents.charttime asc limit 1