From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
The Time Machine (タイム マシン, Taimu Mashin) is a machine built by Future Bulma Brief, the future version of Bulma Brief in Future Trunks Brief's alternate timeline, that could be used to go to the future or past.
When travelling back in time, the Time Machine created a new timeline that branched off from the original one at the moment in time when the Time Machine arrived. Thus, it was not possible to "change" the past in the sense that one's present would change. Any change done in the past would be reflected in a new timeline. However, when travelling back to the future, the Time Machine would go back to its original timeline and not to the new timeline's future. If it was used to travel back again to the past to a point after a previous trip, the Time Machine would arrive in the newest timeline.
According to Future Trunks, the Time Machine used up a lot of energy therefore it could carry barely enough power for two trips in time. Thus, it was generally used to travel back in time once and then to return to the present. Recharging the machine took a lot of time. It seemed that the only way to recharge it is in Future Trunk's own time.
Future Trunks used the Time Machine to go to the past to warn the Z Warriors about the Androids. Because of the way the machine worked, Future Trunks' timeline did not change at all (see Alternate timeline for more details). Future Trunks went back in time not to change his history, but to learn of any weaknesses the Androids may have had. That way, he could return to his time and manage to defeat the Androids there. Future Trunks used the machine again to go back and help the Z Warriors when the Androids made their appearance.
In Future Trunks' timeline, after Future Trunks' return from his second trip to the past and his deactivation of the Future Android 17 and Future Android 18, Future Cell killed him and stole his Time Machine. He did it in order to go back in time to absorb the Androids in a moment in time when they still existed. This essentially created four timelines. The fourth timeline, which was never seen, involved Future Trunks going back in time and, instead of training in the Hyperbolic Time Chamber, finding the blueprints to the Androids. Bulma later crafted the Shut Down Remote, a device that would deactivate them and leave them helpless, and Future Trunks took that controller back to his future time to stop the Androids. Since he had not trained in the Hyperbolic Time Chamber, Future Trunks was not strong enough to kill Future Cell, leading to Future Cell killing him and taking his Time Machine to the past and creating the events that make up the Cell Saga.
When Future Cell reached the past, he abandoned the stolen Time Machine. It was found and photographed by a Capsule Corporation satellite, however, four years later. Bulma showed the photograph to Future Trunks and he decided that they should check it out. They found the Time Machine and Cell's shed cocoon. After inspecting it, Future Trunks realized that it was the same as his own Time Machine (as it had the word 'Hope!!' written on the side). Future Trunks pressed a button to capsulize it and likely took it back with him to his future timeline to prevent anyone from misusing it (or reverse-engineering the technology).
 Other Time Machines
In Dragon Ball Z: Wrath of the Dragon, Bulma created a Time Machine for Tapion to allow him to return to Planet Konats in the past. It was nearly identical to the one Future Trunks used (the design was likely based on Future Trunks' Time Machine). According to the Japanese dialog, Bulma told Trunks Brief that he could use another Time Machine to visit Tapion any time he liked.
 Video games
Time Machines appear in Dragon Ball Online. The game features Time Machine Missions that allow players to travel back in time and take part in notable events in history. The main antagonist, Miira, plans to use a Time Machine to go into the past and steal Goku's DNA.
It is named Time Capsule in Dragon Ball Z: The Legacy of Goku II.
- Artificial Human No. 16 (in the Affectionate Android What If Saga stage in Dragon Ball Z: Budokai Tenkaichi 3)
- Bulma (in Dragon Ball Z: Atsumare! Gokū Wārudo)
- Future Trunks
- Future Artificial Human No. 17 (in Dragon Ball Z: Supersonic Warriors)
- Future Artificial Human No. 18 (in Dragon Ball Z: Supersonic Warriors)
- Kuririn (in Dragon Ball Z: Atsumare! Gokū Wārudo)
- Son Goku (in Dragon Ball Z: Atsumare! Gokū Wārudo)
- Son Gohan (in Dragon Ball Z: Atsumare! Gokū Wārudo and Dragon Ball Z: Supersonic Warriors 2)
- The word 'Hope!!' is written on the side of the machine by Future Bulma, emphasizing that it is hope for their world.