Creating index on concatenated char columns fails is Postgres 9 (regression)

Steps to reproduce:

Run commands

create temp table test (kuupaev date, kellaaeg char(5) ) on commit drop;
create index test on test ((kuupaev||kellaaeg));

in

"PostgreSQL 9.3.4, compiled by Visual C++ build 1600, 32-bit"

Observed result:

ERROR: functions in index expression must be marked IMMUTABLE

In

"PostgreSQL 8.4.4, compiled by Visual C++ build 1400, 32-bit"

those commands work OK.

I need this index to speed up query

SELECT
max( kuupaev||kellaaeg )
from ALGSA
where laonr=?nlaonr and kuupaev <=?prmLOPP and kuupaev||kellaaeg <= ?someparam

How to fix or other way to speed this query?

Posted also in http://stackoverflow.com/questions/26161561/how-to-create-composite-index-in-postgres-9

Andrus.