when was patient 31260 prescribed the drug senna and glyburide at the same time for the first time until 01/2105?
select t1.startdate from ( select admissions.subject_id, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where prescriptions.drug = 'senna' and admissions.subject_id = 31260 and strftime('%y-%m',prescriptions.startdate) <= '2105-01' ) as t1 join ( select admissions.subject_id, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where prescriptions.drug = 'glyburide' and admissions.subject_id = 31260 and strftime('%y-%m',prescriptions.startdate) <= '2105-01' ) as t2 on t1.subject_id = t2.subject_id where datetime(t1.startdate) = datetime(t2.startdate) order by t1.startdate asc limit 1