when did patient 3516 receive the first calculated total co2 lab test until 42 months ago?
select labevents.charttime from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'calculated total co2' ) and labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3516 ) and datetime(labevents.charttime) <= datetime(current_time,'-42 month') order by labevents.charttime asc limit 1