The Warring States period!
As someone who had transmigrated into this world, Uchiha Sora knew the history of the Naruto world very well. Though he had learned little from the members of Black Sand, it was enough to establish that this was most likely the Naruto world's Warring States period.
In other words, the era before Madara Uchiha and Hashirama Senju founded Konoha.
Uchiha Sora inwardly sighed in relief at that conclusion. Thankfully, this was still the same Ninja world, so it was at least connected to the world they had come from.
If they had ended up in a completely unfamiliar world or plane of existence, they would have been in serious trouble.
Afterward, Uchiha Sora and Tsunade asked the members of Black Sand a few more questions, finally gaining some understanding of the world's current situation.
Once they had finished, the two let the members of Black Sand go.
After the members of Black Sand left, Uchiha Sora said to Tsunade, "Lady Tsunade, it looks like we used the Dragon Vein's energy to reverse space-time and reach the Warring States period sixty or seventy years ago. But according to the grandfather paradox, this is more likely a parallel world."
"The grandfather paradox?" Tsunade was taken aback.
Uchiha Sora smiled and explained, "The grandfather paradox is a theory about traveling through space-time. If time travel is possible, then you can only travel into the future, not the past."
"The specifics are what the grandfather paradox describes: if I traveled back into the past and killed my grandfather, I would never be born. And if I were never born, I couldn't exist, nor could I go back into the past to kill my grandfather. That's the grandfather paradox."
"..." Tsunade was speechless.
"That shouldn't be too hard to understand, right?"