Turn postgres date representation into ISO 8601 string

This is a terse way to "turn a PostgreSQL date representation into an ISO 8601 string":

SELECT to_json(now())#>>'{}'

It uses the #>> operator in combination with the to_json() function, which can both be found on this page: https://www.postgresql.org/docs/current/functions-json.html

The operator "Get[s] JSON object at specified path as text". However when you specify an empty array literal '{}' as the path, it specifies the root object.

Compare this method to similar methods:

SELECT
to_char(now(), 'YYYY-MM-DD"T"HH24:MI:SSOF') AS most_lengthy, -- See note: *
trim(both '"' from to_json(now())::text) AS a_bit_lengthy,
to_json(now())::text AS unwanted_quotes,
to_json(now())#>>'{}' AS just_right

It's shorter but produces the same results.

User @atoth pointed out that the subsecond component has its trailing zeros removed using to_json() so 2022-03-31 17:39:23.500 is converted to 2022-03-31T17:39:23.5Z. Since some date recipients require very specific format, I tested the following:

SELECT
to_char('2022-03-31 17:39:23.5'::timestamp, 'YYYY-MM-DD"T"HH24:MI:SS.MSOF'),
to_char('2022-03-31 17:39:23.500'::timestamp, 'YYYY-MM-DD"T"HH24:MI:SS.MSOF'),
to_char('2022-03-31 17:39:23.5123456789'::timestamp, 'YYYY-MM-DD"T"HH24:MI:SS.MSOF'),
to_char('2022-03-31 17:39:23.5123456789'::timestamp, 'YYYY-MM-DD"T"HH24:MI:SS.USOF')

This was the output from Postgres 10:

"2022-03-31T17:39:23.500+00", "2022-03-31T17:39:23.500+00", "2022-03-31T17:39:23.512+00", "2022-03-31T17:39:23.512346+00"

So it appears that with MS you can specify exactly 3 decimals, no more and no less and with US you will get exactly 6, no more and no less. Outside of these two precision options, you'll have to do some funky string manipulation.

Have fun!

* Also, JavaScript will not parse the first method's output via the Date() constructor, because it expects a simplification of the ISO 8601 which only accepts time zones in (+/-)HH:mm or Z format, but OF returns (+/-)HH format without the minutes, UNLESS the input timezone is a fraction of an hour, e.g. using SET timezone=-4.5; at the beginning of the session. Alternatively you could manually append your timezone as a string to the lengthy version and exclude the OF

I think I found a way to do the formatting, but it's not ideal because I'm writing the formatting myself.

Here is a potential solution:

SELECT to_char (now()::timestamp at time zone 'UTC', 'YYYY-MM-DD"T"HH24:MI:SS"Z"')