Sent to Earth as a mere baby to lay preparations for an alien invasion, Goku suffers a clonk on the head, losing all memory of his mission and subsequently becoming a champion defender of our planet. I haven’t watched enough episodes to know for sure when the first time travel occurred, but it may have been in Episode 122 of the second Dragon Ball series (Dragon Ball Z, “My Dad is Vegeta”) in which time traveler Trunks arrives with a warning. Trunk and time traveling continued into the reboot series, Dragon Ball Z Kai, which I’ve seen on the Toon network.