Time doesn’t exist.
Objects exist. Objects move. We can compare one object’s movement with another object’s movement. Time is just “agreement” on which base sample object’s movement to use as reference.
So, what is a timeline?
Fiction.
You would have a “legal” time travel even with a discontinue timeline (aka time teleport), or with a sharp and sudden change of the direction (like turnstiles in tenet)… or even an “illegal” time travel with a plain straight timeline… it all depends on the suspension of disbelief reached in the fiction context.
On Bioshock Infinite this timeline:
feels smooth as a degree 11 nurbs…