Datetime trunc in where clase

I am writing and comparing results of two queries:

select to_char(cast((from_tz(ntimestamp# ,'00:00') at local) as date) ,'YYYY-MM-DD HH24:MI:SS') from sys.aud$

where cast((from_tz(ntimestamp# ,'00:00') at local) as date)  < to_char(trunc(sysdate - 1, 'DD'), 'YYYY-MON-DD HH24:MI:SS')

order by ntimestamp# asc;

TO_CHAR(CAST((FROM_                     

-------------------

2017-08-13 23:59:40

2017-08-13 23:59:40

2017-08-13 23:59:41

2017-08-13 23:59:41

2017-08-13 23:59:41

2017-08-13 23:59:41

2017-08-13 23:59:42

2017-08-13 23:59:42

2017-08-13 23:59:43

2017-08-13 23:59:43

and

select to_char(cast((from_tz(ntimestamp# ,'00:00') at local) as date) ,'YYYY-MM-DD HH24:MI:SS') from sys.aud$