In the physical universe we live in, the concept of time is always positive and always starts at zero – the time of the big bang.  However, quantum physics today postulates that there may be an infinite number of parallel universes and these other universes contain space, time and strange forms of exotic matter. It is also theoretically possible in contemporary physics for a parallel universe to concievably have no time dimension or time can become negative in that environment. The lack of time as a dimension or presence of negative time (or anti-time) makes living through that environment very different form the physical universe. Space can becomes finite and time becomes a non-existent or retraceable factor. As we can walk around in the physical universe from point A to point B, in the parallel universe, it can be possible to walk from one time to the next time, or back in time.

A representation of Time and Anti-Time in
quantum physics

The concept of Anti-Time was explored in the Star Trek: The Next Generation episode "All Good Things....", where an anti-time eruption occured as the result of the convergence of three identical inverse tachyon pulses from three time periods - 2364, 2370 and an unknown future date - at a single point in space. This spatial anomaly was formed as  a result of the breakdown of the subspace barrier between time and anti-time, causing "a multiphasic temporal convergence in the space-time continuum". The rupture, located in the Devron system inside the Romulan Neutral Zone, expanded as it traveled back through time due to the unique nature of anti-time. The energy emitted was in the form of temporal energy, which caused Geordi La Forge's DNA to regenerate, giving him normal sight and several other crew members reported old scars healing. The anomaly's presence at Earth in the distant past would have prevented the formation of the first proteins, and ultimately, all life on the planet.  Eventually, the cause of the rupture was uncovered, and a static warp shell created by the Enterprises of each time frame was successful in reforming the subspace barrier, erasing the anomaly and the timelines it created.