CREATE OR REPLACE
FUNCTION nsoft.tstamp_format(
    tstamp timestamptz,
    tzone text default 'utc',
    mask text default 'YYYY-MM-DD"T"HH24:MI:SS.US"Z"'
)
RETURNS text
LANGUAGE sql
AS $function$

    select ret from (
        select
            set_config('timezone', tzone, true),
            to_char(tstamp, mask) ret
    ) t;
$function$
;