since 6 years ago, when was the last time that patient 18677 was diagnosed with stomatits & mucosits nec?
select diagnoses_icd.charttime from diagnoses_icd where diagnoses_icd.icd9_code = ( select d_icd_diagnoses.icd9_code from d_icd_diagnoses where d_icd_diagnoses.short_title = 'stomatits & mucosits nec' ) and diagnoses_icd.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 18677 ) and datetime(diagnoses_icd.charttime) >= datetime(current_time,'-6 year') order by diagnoses_icd.charttime desc limit 1