Aristotle's explanation of "unity of time" appears in part five of Poetics. Here, Aristotle is explaining the difference between the tragedy and the epic. Within this discussion, Aristotle defines three unities: unity of action, unity of time, and unity of place. Unity of action refers to the tragedy possessing a beginning, a middle, and an end. The play's action must rely solely upon itself (no unexpected characters, abilities, or actions can be introduced to change the action of the play). Unity of place refers to the idea that everything takes place in a singular setting. Unlike the epic, where the setting is vast, the setting of the tragedy is relatively small.
The unity of time refers to the concept that the action of the tragedy tends to take in a single day, or, according to Aristotle, the action of the play should try to "as far as possible, confine itself to a single revolution of the sun."
An example of this concept can be seen in a comparison between the epic Beowulf and the tragedy Macbeth. The action in Beowulf takes place in a span of well over fifty years. When Beowulf is first introduced, he is in the Danelands to help Hrothgar rid Heorot of Grendel. After successfully defeating Grendel (and his mother), Beowulf rules the Geatlands for over fifty years.
Macbeth, on the other hand, does not give a specific time-line. Readers know that some time passes, though through inference alone. Readers can correctly assume that it takes time for Malcolm and Donalbain to travel England and Ireland. This said, given that the play does not name any time, it could be seen as one continuous action.