when did patient 15794 have a first anion gap laboratory test during their current hospital visit?
select labevents.charttime from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'anion gap' ) and labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 15794 and admissions.dischtime is null ) order by labevents.charttime asc limit 1