Time-Conveyor

A  was a time machine invented by Peter and David's uncle.

Overview
Broadly speaking, the Time-Conveyor was a fully functional time machine. Indeed, it was functional enough to have been officially "launched" during a press conference on Earth. Still, it did have some known bugs. Navigation to precise moments in time was not assured, nor would it always return home reliably.

For these reasons, the Time-Conveyor was effectively a system of two machines: the Mark One and the Mark Two. The Mark One was the main unit, while the Mark Two was effectively slaved to it as a rescue pod. The Mark Two could always find the Mark One no matter where the Mark One landed, thus ensuring that the Mark One would never be totally lost.

Even so, the Mark One's unreliability wasn't as profound as might have been expected. The inventor took a number of reporters forward in time six months, then brought them back to the product launch without requiring the Mark Two. (DAN: Timechase)

Dalek theft
In the aftermath of the product launch, Daleks stole the Mark One, hoping to be able to take it back to Skaro so they could reverse-engineer it and make a time machine of their own. However, the inventor's nephews followed the Daleks through time in the Mark Two, and were eventually able to eliminate the Daleks and return the ship back into their uncle's custody. (DAN: Timechase)

Behind the scenes

 * The story gives no indication of what time was "home" for the unnamed uncle and his nephews. It's thus impossible to assert when in Earth's history this invention was discovered.
 * The description of the reliability of the machine has some commonality with the First Doctor's TARDIS. In practise, however, the ship is shown to return home both times the return button was actually flipped.