instruction: what was the first procedure time of the revision of pedicle grft of patient 2004?
response: SELECT procedures_icd.charttime FROM procedures_icd WHERE procedures_icd.icd9_code = (SELECT d_icd_procedures.icd9_code FROM d_icd_procedures WHERE d_icd_procedures.short_title = 'revision of pedicle grft') AND procedures_icd.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 2004) ORDER BY procedures_icd.charttime LIMIT 1