what was the first time that patient 57023 got a microbiology test during a month before?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 57023 ) and datetime(microbiologyevents.charttime,'start of month') = datetime(current_time,'start of month','-1 month') order by microbiologyevents.charttime asc limit 1