I often think about time, and what I think it is defined by, what makes it tick, literally. The thing is, time is really just us marking down the movement of objects in space, and applying all these lines of measurement to it. It's really funny measurements too, they aren't even at all, for example, the number of days in a year is actually an estimate, and to keep it from getting too off they created the leap years!
So I guess that's the first thing about time I want to discuss. The whys and hows of the measurement process are not all that important, though it is a funny thing about our current system for time. What is important to understand is that time is an invention of the minds of humans, as a way to mark the movement of our planet around the sun, and the events of our lives on the earth while it does so. Really it just gives us a frame of reference for that unknowable constant, the present, and it's relationship to the past and future.
The present itself is a bit hard to conceptualize as well. The moment that it is, is just that, and it is gone before you can even think of it as being the present. It's nothing but a marker for the precise moment that divides the past from the future. Really, it defies our ability to define it, and you could argue that it's nothing at all.
You may be wondering then, what time is, if the present is nothing but a marker for the division between the past and the future. Well, I feel like time is bound both by the earth itself, and by our minds. In other words, time did not come to be until we defined it, and through our definition of it, we have bound it to the movement of our planet as well. If you think about it, we have one day for one revolution of the earth, one year for one revolution of the earth around the sun. In the past we also had the months divided by the moon cycles, though now it's a bit more loosely based on that. This means that if humans were on another planet, even in this solar system, their time would be different. I think that if we do end up with multiple planets with human civilizations, we will need to revise this. It's crazy to try to keep track of the minutes and hours and days when you are on a planet that doesn't match up that way, it would get way too confusing. I believe that time has to be bound to the terrestrial home of the intelligent being that is keeping track of it, because this is part of how it is defined for us. Either how we define it or standardize it will need to change, or our understanding as a whole does. Though going to in depth into that is not really important as that hasn't happened yet.
One part about time that is important to define is the future, the unknowable void towards which we are always heading, to the end of time. There is some prediction possible for the future, but there are so many variables that the further from the present you get, the less likely you are to be correct in your prediction. But with divination, the view into time is different than coming up with a prediction based on the likelihood of an event. Divination is using a medium to contact the spirits or energy, however you define it, that exist outside of time and outside of what we know of the physical world. The information about the future that you get through divination is guided by your use of the medium, and your interpretation of the answers given. More than that though it is guided by the energies that come into play. The future seems to be veiled from us, but really it is there all along, and often times will come to pass just as it seems it will. The times when it does not are the important times. When things don't go as expected, that is when you can see the changeable nature of existence, and the amazing adaptability of all life forms.
The past to me is very important, but in the concept of the meaning of time, it's simply memories. Memories written down, or recalled in our own minds. Memories that are passed down in cultures or families. The fact is that the past is only what we remember of it. It's different for everyone, and often times inaccurate.
Time, as a concept is like I said, our invention, the lines we are marking out to keep track of ourselves. In other words, it's our perception of it, in a measurable form. If you allow time to branch out in it's meaning you can think about the perception of it, outside it's measurable (numeric) form. I'm sure most everyone can agree that a period of time can seem longer or shorter than the clock says it is. Sometimes just a few moments can seem like hours, and vice versa.
In using magic to manipulate time, generally I feel like the only part of time that I have been successful in manipulating is the perception of time. I can make the perception of time for myself and sometimes others I am with change, either to seem like more or less time than it really is. Sometimes when I am late I will imagine a clock moving less slowly, but I find it hard to effectively do. I usually would try imagining whatever clock it is that I need to slow, like my supervisor's clock on his computer, or the clock for the receptionist, depending of course on what I am late to. I don't really feel like that type of time manipulation has actually worked for me. The other way that I try to manipulate time in a more indirect manner is through fixing in my mind the need for me to get quickly through obstacles, and that has worked before. Like, having all the traffic lights green, or a car moving out of the way, that type of thing. I think it could also work better to cause a person to mistake the time they see, or to not check what time it is, or not care.
One bit of warning: when I have manipulated time and it had a dramatic effect, I also seemed to have some consequences for my success. Time seems to need a balance, so if you slow time down, you should account for time to speed up again and if you don't want it to do that at a certain point, you should probably assign the balance as well, so if you were slowing time down to get to work, you would want to allow time to speed up to balance itself out afterwards. Otherwise, time might speed up before, meaning you are still late! Or, if you were making something last longer, like a special weekend or trip, you need to account for the time that will be speeding up. So, to make sure that you avoid some twist of time coming back to bite you, account for the opposite of what you intend to accomplish in your spell too.
To sum this all up, I think time is something that is unchangeable, but the perception of that constant, and the manipulation of the people and things that exist in a certain time and place make it possible to change time with magic, but be wary of the balance needed to accomplish time manipulation.