create index on a jsonb timestamp field?

I'm playing with DNSTAP (dnstap.info) data and loading it into a
database for analysis.

when I try to create an index on the query_time field of the json
structure I get:

ler=# select id,data->'message'->>'query_time' from dns_query limit 2;
id | ?column?
----+-----------------------------
2 | 2019-05-13T01:35:59.822984Z
3 | 2019-05-13T01:35:59.829801Z
(2 rows)

ler=# select id,(data->'message'->>'query_time')::timestamptz from
dns_query limit 2;
id | timestamptz
----+-------------------------------
2 | 2019-05-12 20:35:59.822984-05
3 | 2019-05-12 20:35:59.829801-05
(2 rows)

ler=#

ler=# create index dns_query_time_idx on dns_query(((data -> 'message'
->> 'query_time')::text::timestamptz));
ERROR: functions in index expression must be marked IMMUTABLE

Is there any easy way to do this? Or, what would the experts recommend
here?

--
Larry Rosenman http://www.lerctr.org/~ler
Phone: +1 214-642-9640 E-Mail: ler(at)lerctr(dot)org
US Mail: 5708 Sabbia Dr, Round Rock, TX 78665-2106