when was the first time that patient 29410 was prescribed trazodone hcl and ns until 21 months ago at the same time?
select t1.startdate from ( select admissions.subject_id, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where prescriptions.drug = 'ns' and admissions.subject_id = 29410 and datetime(prescriptions.startdate) <= datetime(current_time,'-21 month') ) as t1 join ( select admissions.subject_id, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where prescriptions.drug = 'trazodone hcl' and admissions.subject_id = 29410 and datetime(prescriptions.startdate) <= datetime(current_time,'-21 month') ) as t2 on t1.subject_id = t2.subject_id where datetime(t1.startdate) = datetime(t2.startdate) order by t1.startdate asc limit 1