when did patient 20000 receive the first anion gap test since 166 months ago?
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 = 20000 ) and datetime(labevents.charttime) >= datetime(current_time,'-166 month') order by labevents.charttime asc limit 1