Time as one of the many dimensions of the universe. Although parts of it will probably be proven wrong at one point or another, (Super)String theory is one of the most advanced/accurate theories we have at the moment.
However, in order for some of the more complex calculations to work, we must live in a Calabai-Yau space. If my memory serves me correctly, I read about this general topic (String theory & time) a while ago in a book called "The Elegant Universe". For some reason or another, these Calabai-yau spaces had eleven dimensions, one of them being time.
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Thinking about it, time being a dimension makes perfect sense.
Imagine being in a two dimensional world. (Flat). Being able to travel in 3 dimensions would allow you to pass through any object (by going above or below it). Similarly, thinking of time as allowing you to pass through any object (because at some other instance, the same wall/object will not have been there), reinforces the idea of it just being another dimension.
However, it is unlike spatial dimensions. We cannot see it, and we cannot (as far as I know), go backwards. However, like in the spatial dimensions, we can control how fast we go. The theory of relativity supports this. Imagine travelling in a world with one dimension at speed S. Now, this world is expanded into two dimensions, and you travel diagonally across it. While your speed may still be M, your speed in one direction of the plane will be a fraction of S. (If it is at a 45 degree angle, it will be sqrt(2)S/2). The speed travelled along the initial direction will be inversely proportional to the speed traveled along the new direction. That is; the faster you travel in the new direction, the slower you will travel in the initial one.
Translating this to Space-Time, according to relativity, the faster we travel in the three spatial dimensions, the slower time travels for us, and the slower we travel in the three dimensions, the faster time travels for us.