How to query from the result of a changed column of a table in postgresql

I am working with Presto on Amazon Athena. I have a string time column in a table and now I want to change that time to date time type and then query data for selected dates. Is there a direct way to do so? One way I could think of is 1) add a new column 2) insert values into it with converted date 3) Query using the new column Here I am stuck with the 2nd step with INSERT so need help with that ALTER TABLE "db"."table" ADD COLUMN new_date timestamp -- Need correction in select statement that I don't understand INSERT INTO "db"."table" (new_date) (SELECT new_date from_iso8601_date(substr(timestamp,1,10)) Could some one help me with correction and if possible a better way of doing it?