How time flies.
What is time? Is it a thing? Is it a dimension? A place?
Many, many authors come to grapple with this question in their writing. Any time travel novel must treat time travel as something either magical or scientific. Magical would include books like "A Conneticut Yankee in King Arthurs Court" in which the hero is transported to the past by a blow to the head. Scientific would, of course, include a machine (in fact, they nearly always do) or a theory that had some reasoning behind it- something like "Timeline."
In fact, I'd make the argument that pure science-fiction always uses a machine. Anything else would require something bordering on magic. Which is why Madelaine L'Engle's books "A Wrinkle in Time" and "A Swiftly Tilting Planet" sit squarely between Sci-Fic and Fantasy. "Wrinkle" is more science fiction, but the means by which they travel is still more magical in form than scientific, no matter how much she says otherwise. "A Swiftly Tilting Planet" moves to more spiritual terms and lets time travel be a power that a creature can have. "Many Waters" on the other hand, actually uses a machine for the outward journey, although the journey home is more miraculous than otherwise.
And then, of course, there is God, who is outside of time. Which leads us to wonder, is heaven itself outside of time? Are the saints and the angels outside of time as well?