instruction: when was the first time that patient 015-91239 on this month/31 had the minimum heartrate value.
response: SELECT vitalperiodic.observationtime FROM vitalperiodic WHERE vitalperiodic.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '015-91239')) AND NOT vitalperiodic.heartrate IS NULL AND DATETIME(vitalperiodic.observationtime, 'start of month') = DATETIME(CURRENT_TIME(), 'start of month', '-0 month') AND STRFTIME('%d', vitalperiodic.observationtime) = '31' ORDER BY vitalperiodic.heartrate, vitalperiodic.observationtime LIMIT 1