when did patient 20033 have the last minimum ph value until 09/2104?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 20033 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'ph' ) and strftime('%y-%m',labevents.charttime) <= '2104-09' order by labevents.valuenum asc, labevents.charttime desc limit 1