Time travel is a common dream event. There is rarely a time machine involved. Rather, the dream story simply unfolds in another era of time, past or future. These dreams often represent either the romance or hopes we attribute to moments other than our present. You may find that the dream stems from your desire to identify with particular mores of a time period or to influence certain events.
If you spontaneously dream of going backwards in time, it is most likely a dream of romantic wish-fulfillment. The "good old days" seem to conjure images of heroism, nobility, morality, and social life that, while not altogether accurate, draw our admiration. Often there is a facet of your personality that you feel you could more easily tap into as a pioneer, statesman, damsel in distress, or some other stereotypical image of history.
Most times, the time travel is confluent with your particular image of an archetypal dream character. For example, if you are dreaming a warrior/hero archetype, you may personify yourself in the dream as a medieval knight.