The rule is that time travel is only possible from one identical Universe to another.

That’s why Trunks didn’t change his past, he changed an alternative timeline (that was originally identical to his).

When going to the past, Trunks goes to a universe identical to his own. And by doing this trip, he creates a change. From there, the universes can diverge.

Is that clear? Because it’s getting more complicated.

When Trunks travels from “future” to “past”, he can do it several times as long as he never goes before his previous trip. This to avoid paradoxes. This is a very important rule.

DB Super avoids this by saying he can ONLY travel exactly the same time (20 years) each time. It’s a very good idea because it also prevents paradox. But it’s incompatible with the dates of DBM and DB Dictionary, so we won’t use it.

If a travel goes further in time, another set of (originally identical) universes is used.

And... And Cell did that. This doubled the number of universes to consider.

Note that this explanation is totally compatible with DBZ and DB manga, and even DBS.

The "Usomething" you see in the graph are related to DBM universes, you can ignore them.

Trunks does 3 trips, each time going back and forth from his future to “our” present. Each time he needs to “recharge” the time machine and it’s taking long.

1. Trip 1, from 784 to 764, twenty years in the past, to warn Gokû.
2. Trip 2, from 785 to 767, to help the team, and to find out a way to kill the cyborgs.
3. Trip 3, from 788 to 767+ (date unknown), to tell the team it’s now ok on his side.