instruction: when did patient 3516 first receive a lab test for potassium since 04/2103?
response: 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 LIMIT 1