The 800 Series, Bluebuck, I-Series, Miramar, and Burnside already all have working clocks. Only the 200BX, Grand Marshal, LeGran, and Pessimas have clocks that aren't functional.
The cars with working clocks display the time of the timezone the player is in and not the in-game time, e.g the clock reading 12:00 at the in-game noon.