Why the year is being stored as an astronomical year in date datatype?

postgres=# SELECT date '4714-11-24 BC';
         date         
----------------------
 -4713-11-24 00:00:00
(1 row)

I am using Ivorysql-1.5.
The date style is set to ISO, MDY.
Is this the expected output?

The same test case is used in ora_date.sql but the expected output was diff there.
Following output is taken from ora_date.out

-- Check upper and lower limits of date range
SELECT date '4714-11-24 BC';
          date          
------------------------
 4714-11-24 00:00:00 BC
(1 row)