when did patient 17398 first take his/her tpn on 08/21/2102?
select inputevents_cv.charttime from inputevents_cv where inputevents_cv.icustay_id in ( select icustays.icustay_id from icustays where icustays.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 17398 ) ) and inputevents_cv.itemid in ( select d_items.itemid from d_items where d_items.label = 'tpn' and d_items.linksto = 'inputevents_cv' ) and strftime('%y-%m-%d',inputevents_cv.charttime) = '2102-08-21' order by inputevents_cv.charttime asc limit 1