The first one. Specifically because of wave function collapse (ie. The cat is both alive and dead until the box interacts with the universe)
I either could have or could not have travelled back in time to the year 1927. In our present universe, the wave function collapsed and revealed that I didn't. If I went back in time to 1927, I'd essentially be re-rolling the dice, causing the wave function to collapse again, this time revealing that I did in fact move back in time.
Re-rolling the dice doesn't change the initial roll. It's immutable in the fabric of reality. All I'm doing is creating a new universe in which I did travel back in time.
If I were to then move forward again, I'd be in the new timeline, not the original one. There's no going home again. Which is why Sam Beckett was never able to return home. He spent four seasons creating different universes where one person's life was better at the expense of a bunch of others.