when was the first time that patient 15119 had a minimum po2 in 03/2105?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 15119 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'po2' ) and strftime('%y-%m',labevents.charttime) = '2105-03' order by labevents.valuenum asc, labevents.charttime asc limit 1