[Den-O]

It really is akin to Doctor Who. Time is happening relative to the Doctor's position in it. Regardless of what he's doing the present marches onward around him. But if something happens in the past that's tied to a point in the future/present, it changes. Like Den-O here, the event tied to the present reshapes the present around it.

The Doctor is also not supposed to be able to travel within his own timeline and effect it but that rule kind of got thrown out in Day of The Doctor. Similarly we know later in Den-O that Singular Points can't be altered or at least they aren't supposed to be able to be. But as we know that too changes later.

Short hand: it all get sort of timey wimey.