ERROR: functions in index expression must be marked IMMUTABLE

Hello Experts..

postgres=# create index idx_calls_call_datetime on calls using btree
((call_datetime::date));
ERROR: functions in index expression must be marked IMMUTABLE

bit confused with above error, can any one please help me on this, and if
possible let me explain when this error occurs and what it does. Thanks in
advance.

Thanks & Regards,
M Naveen Kuamr,
PostgreSQL Database Administrator,
Mobile Number: +91 7755929449.