when did the first microbiology testing of patient 28634 take place during their first hospital visit?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 28634 and admissions.dischtime is not null order by admissions.admittime asc limit 1 ) order by microbiologyevents.charttime asc limit 1