type cast in index

Hello,
i have reading in the mailing list any messages where different people use this
format to establish a functional index in a column using a type cast.

CREATE INDEX fecha_creacion_ordtrab ON orden_trabajo_cabecera USING btree
((time_stamp_creacion::date));

but in my postgresql 8.3 version i get this error:

ERROR: functions in index expression must be marked IMMUTABLE

I have tried with the typical CAST too but i get the same error, the column in
the table is:

Column | Type |
Modifiers
time_stamp_creacion | timestamp with time zone | default now()

What can be the problem? Thanks.

Regards,
Miguel Angel.