in this hospital encounter, when patient 15794 was prescribed for the first time for labetalol hcl?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 15794 and admissions.dischtime is null ) and prescriptions.drug = 'labetalol hcl' order by prescriptions.startdate asc limit 1