The Dragon Ball series is no stranger to outlandish science fiction concepts and ideas, and this definitely comes to a head in the Future Trunks arc in Dragon Ball Super. The English language broadcast of the series is midway through the arc, and it's clarified how time travel works within the series.
Naturally, only some characters are allowed to time travel freely lets they tear apart the fabrics of reality.
When Whis and Beerus find out that Future Trunks used a time machine (that Future Bulma built), Whis mentioned that time travel was against the law of the universe and that humans should not be able to do it. Beerus only refuses to destroy Trunks after Bulma treats him to fish sausages, but Goku Black showing up through a time rift throws a wrench into everything.
Whis explained that time travel is indeed allowed, but only to the Gods. Even his power to turn back time is allowed, but only because it goes back three minutes. Any further than that would ruin the flow of a universe. Which is why Gods are the only ones allowed to go back with the use of a Time Ring.
The Time Ring is a divine artifact that allows a Supreme Kai to monitor a universe or sense disturbances in the time stream (which is why Black was brought to the present temporarily). But even then, Kais aren't allowed to interfere.
So there are definite rules in place for time travel, rules that Bulma and Trunks have broken on multiple occasions. There's also a distinct, proper way to time travel, but as long as Beerus and Whis refuse to interfere Bulma's going to have to be a little tricky.
DragonBall Super airs on Crunchyroll Saturday evenings at 7:15 p.m. CST. Adult Swim airs the English dub during its Toonami block Saturday evenings at 9:30 p.m, and is now available to stream on FunimationNOW and Amazon Video.
If you want to catch up with the English dub, the first 39 episodes of Dragon Ball Super are now available to stream on FunimationNOW, Crunchyroll's VRV service, and available to purchase on Amazon Video. The 39 episodes span the full range of what has aired in the North America and covers the "Battle of Gods" arc, "Revival of F" arc, and the most recently ended "Universe 6" arc.
There is also currently a new Dragon Ball film in the works for 2018. The film will focus on the Saiyans, the "origins of Goku's power," and potentially the story of the very first Super Saiyan God. Not only does it aim to be the best film in the series, original creator Akira Toriyama will be contributing to the film's script and new character designs.