when was first the arterial bp [diastolic] of patient 15945 measured?
select chartevents.charttime from chartevents where chartevents.icustay_id in ( select icustays.icustay_id from icustays where icustays.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 15945 ) ) and chartevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'arterial bp [diastolic]' and d_items.linksto = 'chartevents' ) order by chartevents.charttime asc limit 1