[postgresql] Using current time in UTC as default value in PostgreSQL

I have a column of the TIMESTAMP WITHOUT TIME ZONE type and would like to have that default to the current time in UTC. Getting the current time in UTC is easy:

postgres=# select now() at time zone 'utc';
          timezone          
----------------------------
 2013-05-17 12:52:51.337466
(1 row)

As is using the current timestamp for a column:

postgres=# create temporary table test(id int, ts timestamp without time zone default current_timestamp);
CREATE TABLE
postgres=# insert into test values (1) returning ts;
             ts             
----------------------------
 2013-05-17 14:54:33.072725
(1 row)

But that uses local time. Trying to force that to UTC results in a syntax error:

postgres=# create temporary table test(id int, ts timestamp without time zone default now() at time zone 'utc');
ERROR:  syntax error at or near "at"
LINE 1: ...int, ts timestamp without time zone default now() at time zo...

Still another solution:

timezone('utc', now())

These are 2 equivalent solutions:

(in the following code, you should substitute 'UTC' for zone and now() for timestamp)

1. timestamp AT TIME ZONE zone - SQL-standard-conforming
2. timezone(zone, timestamp) - arguably more readable

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

Explanation:

• zone can be specified either as a text string (e.g., 'UTC') or as an interval (e.g., INTERVAL '-08:00') - here is a list of all available time zones
• timestamp can be any value of type timestamp
• now() returns a value of type timestamp (just what we need) with your database's default time zone attached (e.g. 2018-11-11T12:07:22.3+05:00).
• timezone('UTC', now()) turns our current time (of type timestamp with time zone) into the timezonless equivalent in UTC.
  E.g., SELECT timestamp with time zone '2020-03-16 15:00:00-05' AT TIME ZONE 'UTC' will return 2020-03-16T20:00:00Z.

Docs: timezone()

What about

now()::timestamp

If your other timestamp are without time zone then this cast will yield the matching type "timestamp without time zone" for the current time.

I would like to read what others think about that option, though. I still don't trust in my understanding of this "with/without" time zone stuff.

EDIT: Adding Michael Ekoka's comment here because it clarifies an important point:

Caveat. The question is about generating default timestamp in UTC for a timestamp column that happens to not store the time zone (perhaps because there's no need to store the time zone if you know that all your timestamps share the same). What your solution does is to generate a local timestamp (which for most people will not necessarily be set to UTC) and store it as a naive timestamp (one that does not specify its time zone).

Wrap it in a function:

create function now_utc() returns timestamp as $$
  select now() at time zone 'utc';
$$ language sql;

create temporary table test(
  id int,
  ts timestamp without time zone default now_utc()
);

Function already exists: timezone('UTC'::text, now())