I did a similar writeup on Dragon Ball Z’s Cell saga timeline earlier. Just like in Z, Super also had a saga involving Future Trunks and, therefore, another time travel mystery. An in-depth understanding of this timeline isn’t necessary to enjoy the story. Like Z, many people feel there are only two timelines in the Zamasu saga of Dragon Ball Super. I’ll present my theories on how this is a quad-timeline story and how a 5th timeline came into existence.
A very brief recap of the Zamasu saga →
[!INFO] Future Trunks goes back in time because Goku Black is terrorizing his world. Black somehow follows back and fights Goku. Beerus and Whis thought the energy was similar to Zamasu from Universe 10. When Goku, Vegeta, and Trunks go to the future, Black achieves Super Saiyan Rose and is stronger than all Z fighters. With this knowledge, they head back to the present, where Beerus deals with a scheming Zamasu; however, that godly act didn’t erase the future Zamasu because he wore a time ring. After multiple back and forths in the time machine, Zamasu is partially defeated and becomes an ether to be destroyed by Zeno. Whis then takes Trunks and Mai back to their past and fixes everything by talking to the then Beerus, and Trunks and Mai live with another version of them.
All of the concepts of time from the Dragon Ball Z blog still hold. However, one of the rules is of significant importance. We established that when a time traveler goes back the first time, it causes a split in the timeline, after which the traveler can travel back and forth without further changes unless another time traveler is involved.
The thing to note here is that while Trunks can travel freely between his timeline and the show’s (the one we watch) timeline, if any or several of the Z fighters go back with Trunks, that’s considered a new traveler. However, if the new traveler shares context with the old traveler, the timeline of the new traveler is affected because he’s the one gaining otherwise foreign context. So, when Goku, Vegeta, and Bulma travel to the future with Trunks and return, they are the ones with new context, which results in a split.
While such a split does happen, it doesn’t need to occur at the point when the traveler travels back. It could also occur at a point earlier than that, where the normal flow of time would end up resulting in the traveler infringing with time. We’ll revisit this through a flow chart.
This diagram represents time as a sequence of events for each of the timelines created during the Zamasu saga. The main timeline and the Future Trunks timeline are continuations of their named counterparts from the Cell Timeline blog post.

Legend →
The red artifacts are all acts of Gods. The orange text explains situations.
For simplicity’s sake, we’ll refer to the Zamasu that becomes Goku Black as Black, even before he becomes Black. On that note, the following is the explanation of some of the important events across all the timelines →
When the arc ends, the main timeline’s Gowasu places 5 time rings (those associated with splits) in the box and locks them away. However, we explored 2 new timelines apart from the 4 from Cell saga - the Fifth timeline and the New Future Trunks timeline. So what’s the discrepancy?
The New Future Trunks timeline continues the old Future Trunks timeline. This should still be considered as the same timeline because Zeno destroyed that timeline, and Zeno is a complete wild card. It is possible he somehow destroyed the entire segment of Black, which erased everything up to the point when Whis takes Trunks and Mai. Zeno is the CREATOR! So, this is plausible. But in any case, when Gowasu places those rings in the box at the moment in the main timeline, the old Future Trunks timeline is the same as the New Future Trunks timeline. Therefore, the only new timeline is the Fifth timeline, where Black is originally from.
That’s my complete interpretation of the time-related concepts from the Zamasu arc of Dragon Ball Super. Phew!