The time machine comes with a lifetime warranty, but with a twist. It is warranted for the lifetime of its inventor.
The inventor wanted to make sure that the grandfather paradox would not apply to himself. So he put lockouts in the control system, to prevent any attempt to jump to before the inventor's birth.
Perhaps the software lockout is reinforced by a hardware lockout, such as this:
- The control system includes quantum entangled qbits.
- Some of the qbits are part of a positronic brain.
- Their twin qbits are part of a regular electronic brain.
- The inventor sought out quantum entangled qbits that use positron-electron pairs that were made on his birthdate in a MeV (Mega Electron Volt) particle accelerator. (Somehow somebody stored and kept track of these particle accelerator products.)
- If you go back to before the inventor's birthdate, each pair of critical qbits is replaced by a 1 MeV photon.
As Michael Kjörling♦ points out, the time machine needs a lot of power. In particular, it needs a lot of power to open the doors. This power is obtained via a controlled matter-antimatter reaction. An electronic brain controls the matter side of this reaction; the positronic brain controls the antimatter side. The two are linked via the quantum entangled qbits. If you go back too far, the link between the two brains dissolves, and you cannot open the door. At the story author's option, if you then return to the valid operating range of the time machine, the link is reinstantiated, and you can open the door.
The positronic brain is not a user-serviceable part. If the user tries to physically tamper with it, the antimatter is likely to be catastrophically released.
As a bonus (for the inventor), the warranty terms give customers an incentive to extend (rather than reduce) the inventor's lifespan.