what was the first procedure time of the revision of pedicle grft of patient 2004?
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 asc limit 1