when was the last time that patient 1902 has the minimum bicarbonate since 47 months ago?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 1902 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'bicarbonate' ) and datetime(labevents.charttime) >= datetime(current_time,'-47 month') order by labevents.valuenum asc, labevents.charttime desc limit 1