what's patient 77980's average value of red blood cells until 33 months ago?
select avg(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 77980 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'red blood cells' ) and datetime(labevents.charttime) <= datetime(current_time,'-33 month')