when did first patient 59220 have the minimum value of magnesium during the current hospital visit?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 59220 and admissions.dischtime is null ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'magnesium' ) order by labevents.valuenum asc, labevents.charttime asc limit 1