cast not IMMUTABLE?

Hi all,
I have a table with ~ 3e+6 rows on it.

I do select on this table in this way:

(1) select * from user_logs where login_time::date = now()::date;

consider that login_time is a TIMESTAMPTZ with an index on it.

If I use the select in this way:

select * from user_logs where login_time = now();

the the index is used.

I'm trying to use define and index in order to help the query (1):

test# create index idx on user_logs ( (login_time::date) );
ERROR: functions in index expression must be marked IMMUTABLE

why that cast is not considered IMMUTABLE ?

How can I define an index for the query (1) ?

Regards
Gaetano Mendola