Ok first before I attempt to explain a theory on how people can see into time let me describe the dimentions of space and time, at least the jist of it:

If we start at point and draw a line we have length, draw another and we have width. A third line makes depth and thus we have the cross section of a cube or 3D figure. Time is in the other dimentions. According to theory all things are in perpetual motion, this is true time. All things that exist do so in a state of continueing change an object is not static. So with this we can form a forth dimention of duration. To what ever extent something endures is called an occurance. As an object occurs its in a state of change. Meaning a transition from one occurance to another. The begining of a new occurance is called a re-occurance.

so we have:
1. length
2. width
3. depth
4. duration
5. occurance
6. re-occurance.
So we can assume that there are at least six dimentions to our reality. From this we can gather that if time is linear then the past was an occurance the presant is occuring and the future will occur. This is how we as humans percieve time in reality, but only as we experince it. We exist now in the presant, a state of eternal change, from a previous state, the past, and are moving into another, the future.

But a human may not have to "see" time as linear. Our human perception is not limited by what we have, do, or will experience. We have an imagination and the use of logic to deduce in certain situations a possible out come. For example a man goes to a bar and sits down to drink. If he were to stop in the presant and look at what he was doing he could infer that he would leave the bar with alcohol in his system. Thus he could forsee the possible out comes of doing so. This isn't precognition but this is how precognition works. Imagine that if humans are capable of seeing past all occurances to see a future/past occurance as it may happen or did happen. This is called the psionic wave front. Our perception of this is that we can only infer possible out comes by seeing an event in time and considering possiblities for out comes. Yet if one could see past this "wave" in theory one could then see an occurance in time as it is encountering various obsticels.

The thoery this comes from uses this metaphor to better describe pre-cognition:
Imagine a lake and on one side a toy boat is released. The waves of the lake are time (a series of occurances) as it is coming toward the shore on the other side of the lake. Assuming the waves will eventually take the boat to the other side then a very small man on the other side looking across can see over the waves and see various things the toy boat will encounter- floating logs, rocks, a beaver. How the man sees over/through the waves is tricky. Perhaps the waves hit a rock and split creating a kind of hole by which the tiny man can see into. Inversely Seeing into the past is like the tiny man setting a toy boat lose at one end and watching it as it encounters various occurances.

The boat and objects it encounters represents an occurance in time. An event is an when something occurs. The lake is our universe. The shore where the man is at is the presant. The waves is our perception of existance working towards the presant. The waves of the lake are high enough that most people could not see over them. As the boat as an occurance travels from one side of the lake to the other it will be affected by running into things that might be in the lake. These are random occurances that effect the boat as it is moving towards the presant. The man is a person who is able to see over/through the psionic-waves of the lake of existance to catch a glimse of an occurance in time as it is affected by things it encounters, other occurances as events, and thus this can be taken to mean a person can see the future of an event and various things that will affect it. To see the past a person finds an event or something that has occured and then sees the various things that effected it (Like psychics that solves murders)

Two examples of clairvoyance would be "20000 leagues under sea" (sub-marines) and the story of the "Titan". The author described detials of a boat that sets sail from england and wrecks in the atlantic. He is frighteningly accurate about how many people died, the dimentions of the boat, the lack of life boats, location of wreck ect, ect.

So somebody like Nostradamous or Plato must have been a taller than usual person on the psychic scale of things. The further one can see into time the "taller" they must be or something they encountered (like halucinagens) made them taller like putting lifts in their shoes. Or they somehow mannaged to see through the waves of the lake (halucinagens!!)

This leads to the idea of can or do things exist in different dimentions of time? If so than time travel is perfectly possible. Though I could imagine that perhaps we can only travel, like put one object in the presant into a different point in time, in one direction that being towards the future or into the past because the future always ends in the presant and the past will never happen again (thus its never changing), though we could view it. I can also imagine that perception of different times would be radically different than how we view the presant. If it is possible to go from one point to another in time like we can go from one point in a sphere to another then are there beings that can do this already? Then I can't even imagine how to communicate with such beings with out first learing how to travel in time just to find a perceptible creature in time. Unless they find us first.

To draw on the previous metaphor:
If the man sees an occurance (a toy boat) in reallity (a lake) before it reaches the shore (the presant) then would it not be possible for the man to get in his row boat and intercept the toy boat and alter its course on the lake? Thus changing an event in time? Perhaps so far as to control the presant reality.

This leads me to think so many things at once that I believe I have reached a good stopping point for now. I will compose my other thoughts later and hope fellow readers will add their opinions and comments.