when did patient 6440 first receive his or her ptt lab test in 12/2103?
select labevents.charttime from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'ptt' ) and labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 6440 ) and strftime('%y-%m',labevents.charttime) = '2103-12' order by labevents.charttime asc limit 1