when was a medication prescribed for the first time for patient 94316 in 03/2103?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 94316 ) and strftime('%y-%m',prescriptions.startdate) = '2103-03' order by prescriptions.startdate asc limit 1