when was the first time patient 78221 was prescribed medicine via po route until 08/2101?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 78221 ) and prescriptions.route = 'po' and strftime('%y-%m',prescriptions.startdate) <= '2101-08' order by prescriptions.startdate asc limit 1