Some people live their life on a timer where everything starts at zero as if they have a certain amount of things to accomplish within a limited time. Other people take their time, not worrying about how long it takes them, but about the quality of the time they put into doing something. No matter the approach, they both see time as being a straight line from A to B that never deviates from its inexorable path.
Some people believe in reincarnation, returning to lead a succession of lives until they complete whatever it was they were meant to complete, or learn what they needed to move on to somewhere else. They still see time as a line that inches forward entropically eating up the ages with an insatiable appetite — otherwise how could they progress and grow?
Time is the barrier against which humankind finds itself running into whenever we consider travel beyond our own little planet's sphere of influence. Even the closest star -- our Sun -- is an unobtainable objective due to, for one, our limited time alive. No matter what system you use to measure time, what calendar you follow, doesn't change the fact that it takes more then a human's life span to make it there, never mind back again.
Science fiction writers have spent as much creative energy on figuring out ways to circumvent the problem of overcoming time as they have on creating plots and character. If a story features space travel and humans, writers feel compelled to show off their make believe physics to justify their characters crossing distances that take more then their life spans. While there have been some interesting theories in that regard suggested, everyone still accepts the premise of linear time.
Things aren't quite as straightforward in Jon Courtenay Grimwood's book Stamping Butterflies, released in Canada by Random House Canada's Bantam/Spectra imprint. While it's not the first novel to take place in the past, the present, and various futures, it also explores the possibility that time isn't necessarily linear. In fact, things could be construed as confusing at the beginning as we try to get our bearings in three separate eras, but Grimwood ties it all neatly together in the end.