PostgreSQL function to output an ISO 8601 formatted timestamp with time zone

iso_8601_timestamp.sql

/* Tested on PostgreSQL 9.6 */

CREATE OR REPLACE FUNCTION iso_8601_timestamp(datetime TIMESTAMP WITH TIME ZONE) 
  RETURNS VARCHAR AS $$
    BEGIN
    RETURN to_char(datetime::TIMESTAMPTZ AT TIME ZONE 'UTC', 'YYYY-MM-DD"T"HH24:MI:SS"Z"');
    END;
  $$
LANGUAGE PLPGSQL;

/* Example Usage */

SELECT iso_8601_timestamp(now());
-- 2018-02-15T20:07:12Z

SELECT iso_8601_timestamp('2018-02-15');
-- 2018-02-15T05:00:00Z