when did patient 3516 first receive a lab test for potassium since 04/2103?
select labevents.charttime from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'potassium' ) and labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3516 ) and strftime('%y-%m',labevents.charttime) >= '2103-04' order by labevents.charttime asc limit 1