when was the last laboratory test in 12/2105 for patient 40707?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 40707 ) and strftime('%y-%m',labevents.charttime) = '2105-12' order by labevents.charttime desc limit 1