when was patient 12927 prescribed a magnesium citrate and diazepam at the same time for the first time during this 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 = 'magnesium citrate' and admissions.subject_id = 12927 and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-0 month') ) as t1 join ( select admissions.subject_id, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where prescriptions.drug = 'diazepam' and admissions.subject_id = 12927 and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-0 month') ) as t2 on t1.subject_id = t2.subject_id where datetime(t1.startdate) = datetime(t2.startdate) order by t1.startdate asc limit 1