when was the first microbiology test of patient 28447, in this month?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 28447 ) and datetime(microbiologyevents.charttime,'start of month') = datetime(current_time,'start of month','-0 month') order by microbiologyevents.charttime asc limit 1