what was first lab test time a month before for patient 5142?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 5142 ) and datetime(labevents.charttime,'start of month') = datetime(current_time,'start of month','-1 month') order by labevents.charttime asc limit 1