Convert it from a string literal using TO_DATE():
SELECT TO_DATE( '2000-01-01 12:00:00', 'YYYY-MM-DD HH24:MI:SS' ) FROM DUAL;
Or use a TIMESTAMP literal:
CREATE TABLE date_table(
date_value DATE
);
INSERT INTO date_table ( date_value ) VALUES ( TIMESTAMP '2000-01-01 12:00:00' );
Oracle will implicitly cast a TIMESTAMP to a DATE when storing it in a DATE column of a table; however you can explicitly CAST() the value to a DATE:
SELECT CAST( TIMESTAMP '2000-01-01 12:00:00' AS DATE ) FROM DUAL;