postgresql - PgSQL turning day-of-year back into date -

i trying determine how turn day-of-year date in pgsql. when this

select date '2013-01-01' + interval '53 days' 

i timestamp:

"2013-02-23 00:00:00" 

so how come when of following

select extract(date (date '2013-01-01' + interval '53 days'))  select extract(date (select date '2013-01-01' + interval '53 days')) 

i "error: timestamp units "date" not recognized"? besides why, how can want, date portion of result of original operation?

use

select (date '2013-01-01' + interval '53 days')::date 

or

select cast(date '2013-01-01' + interval '53 days' date) 

postgresql's standard sql function "extract()" will operate on timestamps, a) "date" isn't valid argument extract(), , b) returns subfields, not collection of subfields. conceptually, date consists of collection of 3 subfields: year, month, , day.

select extract(year current_timestamp),        extract(month current_timestamp),        extract(day current_timestamp),        -- concatenate , cast type "date".        (extract(year current_timestamp) || '-' ||         extract(month current_timestamp) || '-' ||        extract(day current_timestamp))::date