when was the last time when patient 46528 was prescribed sevelamer and erythromycin at the same time in 04/2102?
select t1.startdate from ( select admissions.subject_id, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where prescriptions.drug = 'sevelamer' and admissions.subject_id = 46528 and strftime('%y-%m',prescriptions.startdate) = '2102-04' ) as t1 join ( select admissions.subject_id, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where prescriptions.drug = 'erythromycin' and admissions.subject_id = 46528 and strftime('%y-%m',prescriptions.startdate) = '2102-04' ) as t2 on t1.subject_id = t2.subject_id where datetime(t1.startdate) = datetime(t2.startdate) order by t1.startdate desc limit 1