when was the first time that patient 7959 had the pco2 maximum value?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 7959 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'pco2' ) order by labevents.valuenum desc, labevents.charttime asc limit 1