Time has not yet been precisely defined as there's no real physical link or known relationship from the concept of a progress of underlying physical events/progression and time.
A physical event in this context and as mentioned elsewhere is according to the thesis an interaction between the Principal Wave and a wave/mass resulting in some kind of action/result on the smallest scale the theory foresees.
The reason to why time is used in physics as a fundamental "topic" and parameter is likely due to the need to satisfy social life that use time as a measured lapse of past events and progression in an orderly manner. Time as a concept is not driving or controlling anything in physics according to the thesis, but another mechanism is controlling the events that lead to the "time" we observe as a consequence.
"Time" does surely relate to underlying events somehow and is important as a tool for observations and basic physics as a counter at least. Because time is not the driving force of the events, time will have anomalies according to the underlying physics and events therein.
Time is neither a "4th Dimension" as the events that do occur is prior (or more basic) to "time" in the chain, dictating that any state in the 3D space has an event-state per event that occurs. Events dictates the actual succession (progression?) and the 3D location the state of that event/succession. Time is a derivate from this as an independent measurement based on time measured from a separate event sequence (that may or may not align with the time/events one try to measure).
From the above it may be obvious that time is in fact a kind of counter on the typical event rate or aggregate that the Principal Force exerts on itself and other waves and "stuff" it hits/forwards. The Principal Force waves do have a frequency that dictates the event rate and any event will depend on this or a fraction/multiple of the same to instantiate itself. This fact will likely have a major impact on quantum mechanics behaviour.
In the case of speed of light, time may be determined by measurement of clocks, but this hides that light is transported via the Principal Force waves having a likely fixed frequency in vacuum for example. The light can only go as fast and long as the number of events and the distance each Principal Force event allows it to move.
In popular physics we know that the speed of light is reduced in solids and gases. This is explained eq. by the refractive index and interactions with "mass". According to this thesis the reduction of the speed of light outside vacuum is caused by changes in the Principal Force properties outside vacuum. A higher density of Principal Forces waves will require the light to rely on more forwarding events to pass through the non-vacuum materials resulting in an increased number of forward events to be executed for the same distance of travel. This will "slow down" the light from an external vantage point, but the light may have the same forwarded rate anyway - it simply has to pass through more of them. Other properties of the Principal Force waves will also affect the count and distance of these "forwarding" events.
According to current science nothing can travel faster than 300,000 kilometres per second (186,000 miles per second). Only massless particles, including photons, which make up light, can travel at that speed.
In simplified Principal Force terms the speed of light likely reflects the maximum forwarding event rate and distance that the Principal Force waves can provide for the photons, while any waves/radiation that cause acceleration style effects from the events will have its speed reduced, as some events are consumed by the acceleration forces, rather being used as forwarding events.
The vacuum situation provides the fastest Principal Force forwarding rate, while one may in theory create a "negative vacuum" situation beyond removing any foreign gas/matter by lengthening the Principal Force waves forward distance and/or increase the forward rate.
Social terms and concepts such as "travel back in time" makes no sense, unless the underlying events can be reversed, which they cannot in the mind of the author. "Travelling forward in time" is not likely, but seems more plausible however as indicated above. Especially if discussing very short forwarded travel times, compared to the full travel time.
So, in short "time" is a counter for Principal Force wave events, which follow the frequency of the same waves. Apart from this it has no relevance in the core of physics.
Light is likely a "passenger" wave, using the Principal Force waves for travelling over distance. Other waves share the same properties. Light is of social/human importance, but is nothing special in the core of physics. Principal Force theory does not rule out that other waves may exist.