How to query from the result of a changed column of a table in postgresql
I was able to figure out as 

select *
from 
(
  SELECT from_iso8601_date(substr(timestamp,1,10)) as day,*
  FROM "nds"."unacast_sample_august_2018"
 )
WHERE  day &gt; date_parse('2018-08-26', '%Y-%m-%d')  
 limit 10