Zack Snyder's Justice League will see the DCEU's version of The Flash use time travel as part of the massive story. One of the biggest storytelling devices in superheroes is time travel that is used for good and bad. When it comes to the Scarlet Speedster, time travel is a key element in Flash's mythology whether it's Barry Allen, Wally West, Jay Garrick, or Bart Allen.
It has been known for quite some time that the Justice League Snyder Cut would pull time travel into the story. Audiences first met Miller's Flash in a brief but pivotal scene in Batman v Superman: Dawn of Justice. Barry's ability to change the timeline was one of the more important things erased from the theatrical cut of Justice League in 2017. But through Snyder's original vision of the movie, The Flash will be seen altering time to some degree.
While the Arrowverse's The Flash has established how speedsters go about doing time travel, the DCEU isn't necessarily doing it the same way. Snyder has set up a new structure for how the DCEU's Barry travels in time. Batman v Superman: Dawn of Justice was a glimpse of that and might be very relevant for The Flash movie from director Andy Muschietti that will use Flashpoint as inspiration (depending on how close to Snyder's canon it is). This is how The Flash time travels in both Batman v Superman and the Snyder Cut of Justice League Snyder Cut.
How Flash Time Travels in Batman v Superman
When Barry abruptly shows up in Bruce Wayne's cave in Batman v Superman, he is coming through a portal while informing Batman that Lois Lane is the key. But how did Barry get there and where (or more precise, when) is he from? This Flash is a 5-year older version of Barry that comes directly from the Knightmare timeline where Darkseid has taken over Earth. The way Barry traveled to the past was through the infamous cosmic treadmill that has existed in The Flash mythology since 1961. It allows speedsters to travel in time and it's part of the DCEU's version of the fastest man alive. In the Snyder-mythology, Batman and Cyborg had built it for Barry so he could go into the past to warn the Justice League so they could prevent the Knightmare future.
Snyder has confirmed Jared Leto's Joker (who is now part of the Knightmare timeline) stole a Mother Box to help them create the machine. The biggest function of the cosmic treadmill is where it's placed before someone starts using it to travel in time. The reason why Future-Barry shows up in Bruce's Batcave was that the machine was in the same location but in the future. If the cosmic treadmill had been placed in the Kent Farm, for example, Barry would have ended up at the same place, but either five years later or ago. If Barry jumped to the wrong point in time, he might get stuck in the void of space where planet Earth used to be. This helps raise the stakes a lot for the DCEU's method to have The Flash traveling in time.
How Flash Time Travels in Justice League
Snyder has revealed already that in one way or another, The Flash will time travel in the Snyder Cut of Justice League too. Although the treadmill hasn't been built yet, Barry will still be able to play with time in some capacity. The biggest difference is that Barry won't have to go back as far as 5 years into the past like his Knightmare counterpart one day does. At this point in Barry's superhero career, he hasn't time-traveled and the Snyder Cut would be the first time he demonstrates that ability. It's worth noting that every time Barry runs, he does still slow time down where it also allows him to alter it, even if it's just a small change. But Snyder is setting up The Flash to be doing something massive in relation to time-travel.
The way that Barry rewrites reality is that if he pushes himself to run at his fastest, it gives him a do-over. Barry could be running so fast that time not only slows down, but it stops as he, essentially, rewinds what has just happened. In the recent trailers, we've seen Barry running towards a massive explosion as he enters the speed force and the blast actually moves inward and ground reassembles in front of his feet, showing he's changing time by a matter of minutes or seconds to reverse some event. This will lay the foundation for the Knightmare timeline (which would be in Snyder's Justice League 2 or 3) where he makes the jump back 5 years.
Justice League Could Set Up the Multiverse Before The Flash Movie
When it comes to The Flash in the Snyder Cut, the most that fans will see him do is alter time by a few minutes at most. His solo film will be where he travels further into the past (and maybe even the future), but also where Barry messes with the Multiverse. However, that doesn't mean Justice League might not do something with the Multiverse to some degree. As Barry discovers that he can alter time, it wouldn't be out of the question that he starts to wonder if running at maximum speed could take him to a different dimension even. When Miller's Flash appeared in Crisis on Infinite Earths, his Barry even referenced that he had theorized about the Multiverse with Cyborg.
That could imply that at some point in the Justice League Snyder Cut, the idea of a Multiverse is at least present through conversation. It's highly unlikely that Barry travels to another Earth in the midst of Steppenwolf trying to bring the Mother Boxes together. Snyder's film was never about exploring other realities and more about the conflict between Apokolips and Earth. The Flash will be where the idea is more explored in-depth as it also gets Michael Keaton's Batman into the DCEU. It's not impossible, though, for maybe a neat DC Easter egg to be present to give the DC Multiverse a soft setup.
The theatrical cut never gave Miller's Flash the shine that he was obviously meant to have in Snyder's Justice League. While the wait has been long, the original vision for this incarnation of the DC team is finally being realized that will now also include an imperative part of Barry's intended storyline. Since the Arrowverse has featured a lot of time travel for the last couple of years, it'll be a treat to see how the DCEU fully does it through Ezra Miller's version of The Flash in Zack Snyder's Justice League.
