[pg104@dbserver ~]$ psql -d postgres
psql (10.4)
Type "help" for help.

postgres=# select version();
                                                 version                                                 
---------------------------------------------------------------------------------------------------------
 Postgresql 10.4 on x86_64-pc-linux-gnu,compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-28),64-bit
(1 row)

postgres=# select pg_postmaster_start_time();
   pg_postmaster_start_time    
-------------------------------
 2018-05-23 15:34:15.448087-04
(1 row)

postgres=# select current_timestamp;
       current_timestamp       
-------------------------------
 2018-05-24 01:57:24.582325-04
(1 row)

postgres=# select date_trunc('second',current_timestamp-pg_postmaster_start_time());
 date_trunc 
------------
 10:24:04
(1 row)

postgres=# select date_trunc('minute',current_timestamp-pg_postmaster_start_time());
 date_trunc 
------------
 10:24:00
(1 row)

postgres=# select date_trunc('hour',current_timestamp-pg_postmaster_start_time());
 date_trunc 
------------
 10:00:00
(1 row)

postgres=# select date_trunc('day',current_timestamp-pg_postmaster_start_time());
 date_trunc 
------------
 00:00:00
(1 row)

postgres=#

https://www.postgresql.org/docs/current/static/functions-info.html