when did patient 14203 first had the minimum magnesium level until 02/2105?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 14203 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'magnesium' ) and strftime('%y-%m',labevents.charttime) <= '2105-02' order by labevents.valuenum asc, labevents.charttime asc limit 1