If rate of passage of time can be varied, or influenced, by other physical phenomena, it is conceivable that a negative delta can be achieved somehow. The real hard part is the implications of that action. For instance, at relativistic speeds, you could actually slow the time inside a capsule where the time elapsed inside the capsule is much shorter than the time elapsed for an external observer, say people on Earth. Now that capsule has not really gone forward in time, but for all practical purposes it appears like it did. Since we perceive time in a linear fashion, this isn't hard for us to grasp. But, what would happen if somehow a device was built that can actually travel backwards in time? Will the actions of those that move backwards in time impact the future? There is an inherent paradox there, but it is only a paradox because our minds are not wired to understand the true nature of time. We are three-dimensional beings, and if time is indeed a fourth dimension, there is no way we can understand this concept. No more than a rectangle or a triangle can comprehend what a pyramid is.