when was the first time, patient 62977 was prescribed 1/2 ns until 02/2105?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 62977 ) and prescriptions.drug = '1/2 ns' and strftime('%y-%m',prescriptions.startdate) <= '2105-02' order by prescriptions.startdate asc limit 1