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 a bonus (for the inventor), the warranty terms give customers an incentive to extend (rather than reduce) the inventor's lifespan.