instruction: when until 6 months ago did patient 031-9128 get the last microbiology test?
response: SELECT microlab.culturetakentime FROM microlab WHERE microlab.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '031-9128')) AND DATETIME(microlab.culturetakentime) <= DATETIME(CURRENT_TIME(), '-6 month') ORDER BY microlab.culturetakentime DESC LIMIT 1