If you were going on a job interview and had to take time off your present job, would you tell your present employer the reason you needed off?

i am actually not going on an interview, i am taking a civil service test for another job. i am wondering if it would be beneficial to tell my boss that i am looking for another job, or tell him a lie about why i need time off. i am debating on how this might affect the way he treats me or thinks of me.