instruction: when did patient 739 last have pco2 minimum since 12/2104?
response: SELECT labevents.charttime FROM labevents WHERE labevents.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 739) AND labevents.itemid IN (SELECT d_labitems.itemid FROM d_labitems WHERE d_labitems.label = 'pco2') AND STRFTIME('%y-%m', labevents.charttime) >= '2104-12' ORDER BY labevents.valuenum, labevents.charttime DESC LIMIT 1