when has patient 19175 had a microbiology test for the first time during a month before?
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