when did patient 28447 have the first maximum calculated total co2 value since 11/2105?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 28447 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'calculated total co2' ) and strftime('%y-%m',labevents.charttime) >= '2105-11' order by labevents.valuenum desc, labevents.charttime asc limit 1