Creating an index on a timestamp with time zone cast to a date-- possible?

Seems you can't do this:

create index on someschema.sometable ((modified_at::date));

If modified_at is a timestamp with a time zone:

ERROR: functions in index expression must be marked IMMUTABLE

But you can if it's a timestamp without a time zone. Anyway, any way of
casting that as to use it as an index?

--
Wells Oliver
wells(dot)oliver(at)gmail(dot)com <wellsoliver(at)gmail(dot)com>