when was patient 3929 for the first time prescribed a drug via ou route until 01/2105?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3929 ) and prescriptions.route = 'ou' and strftime('%y-%m',prescriptions.startdate) <= '2105-01' order by prescriptions.startdate asc limit 1