From Dragon Ball Encyclopedia
In the alternate future timeline, Future Freeza and Future King Cold journey to Earth to destroy it in August of Age 764. But before they could even begin their assault, Future Goku returned to Earth via his Instant Transmission technique and successfully killed both Future Freeza and Future King Cold. It was at this point that King Cold and Freeza had their DNA sampled by Future Dr. Gero's Remote tracking device for the creation of Cell.
- Interesting enough, in the episode "His Name is Cell," it is implied by Cell, that Future Trunks is the one responsible for the death of Freeza and King Cold in his timeline.