Formatting a date as ISO-8601 in PostgreSQL

The magic string is 'YYYY-MM-DD"T"HH24:MI:SS.MS"Z"'.

SELECT
    NOW(),
    TO_CHAR (NOW() at time zone 'UTC', 'YYYY-MM-DD"T"HH24:MI:SS.MS"Z"')

Hack way using TO_JSON:

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

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