what was the first ph laboratory test time patient 31482 had during this month?
select labevents.charttime from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'ph' ) and labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 31482 ) and datetime(labevents.charttime,'start of month') = datetime(current_time,'start of month','-0 month') order by labevents.charttime asc limit 1