when in 05/this year was patient 3939's last test for po2?
select labevents.charttime from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'po2' ) and labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3939 ) and datetime(labevents.charttime,'start of year') = datetime(current_time,'start of year','-0 year') and strftime('%m',labevents.charttime) = '05' order by labevents.charttime desc limit 1