when had patient 83901 first been prescribed for vancomycin and pantoprazole at the same time during the last month?
select t1.startdate from ( select admissions.subject_id, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where prescriptions.drug = 'vancomycin' and admissions.subject_id = 83901 and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-1 month') ) as t1 join ( select admissions.subject_id, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where prescriptions.drug = 'pantoprazole' and admissions.subject_id = 83901 and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-1 month') ) as t2 on t1.subject_id = t2.subject_id where datetime(t1.startdate) = datetime(t2.startdate) order by t1.startdate asc limit 1