what was the first sulfameth/trimethoprim ss prescription time for patient 51663 during their first hospital encounter.
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 51663 and admissions.dischtime is not null order by admissions.admittime asc limit 1 ) and prescriptions.drug = 'sulfameth/trimethoprim ss' order by prescriptions.startdate asc limit 1