when did the last month patient 19175 have been given a microbiology test for the first time?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 19175 ) and datetime(microbiologyevents.charttime,'start of month') = datetime(current_time,'start of month','-1 month') order by microbiologyevents.charttime asc limit 1