The prosecution will present calibration logs and security tampering prevention information to show that the device was independently verified as working correctly and demonstrably unchanged from that inspection date. A lot of people's careers rely on those records being correct, up to including a perjury charge if they're falsified.
IANAL. I had this talk with a lawyer friend a few weeks ago. I thought there was a way to show the design of the device is faulty - for example a speed gun at sunset reports inaccurate times. But it's a very high standard to get a court to allow that.
