when was first time that patient 20000 had the minimum calculated total co2 value since 102 months ago?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 20000 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'calculated total co2' ) and datetime(labevents.charttime) >= datetime(current_time,'-102 month') order by labevents.valuenum asc, labevents.charttime asc limit 1