when was the first time when patient 99647 had the minimum value of mch on this hospital encounter?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 99647 and admissions.dischtime is null ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'mch' ) order by labevents.valuenum asc, labevents.charttime asc limit 1