Try a proper date literal:
select last_name,hire_date
from employees
where hire_date < DATE '1990-01-01'
Here's a running example: http://sqlfiddle.com/#!4/782c0/1
CREATE TABLE Employees (
last_name VARCHAR(50),
hire_date DATE
);
INSERT INTO Employees VALUES ('A', DATE '1985-01-01');
INSERT INTO Employees VALUES ('B', DATE '1986-01-01');
INSERT INTO Employees VALUES ('C', DATE '1987-01-01');
INSERT INTO Employees VALUES ('D', DATE '1988-01-01');
INSERT INTO Employees VALUES ('E', DATE '1989-01-01');
INSERT INTO Employees VALUES ('F', DATE '1990-01-01');
INSERT INTO Employees VALUES ('G', DATE '1991-01-01');
INSERT INTO Employees VALUES ('H', DATE '1992-01-01');
INSERT INTO Employees VALUES ('I', DATE '1993-01-01');
INSERT INTO Employees VALUES ('J', DATE '1994-01-01');
INSERT INTO Employees VALUES ('K', DATE '1995-01-01');
INSERT INTO Employees VALUES ('L', DATE '1996-01-01');
INSERT INTO Employees VALUES ('M', DATE '1997-01-01');
INSERT INTO Employees VALUES ('N', DATE '1998-01-01');
select last_name,hire_date
from employees
where hire_date < DATE '1990-01-01'