In the Dark Ages of the Western World, the clocking of the passage of Time was understood to be a qualitative endeavour, an effort to explain the significance of the moments, hours and days lived. Time was a form of significant shape, and was often expressed in terms of Cycles. Today the concept of Time is radically different: Time can be measured linearly, and the more accurate the measurement the more Time is presumed to be understood (or, at the very least, commanded). It might be argued that the inherent idea of Time which operates in Science Fiction is the modern idea and that the concept of Time which governs fantasy is the medieval concept.
This sense that, for the medieval mind, Time enfolds the shape of events is analogous to the sense that, in fantasy texts, Story expresses the shape of events. In both cases, what is significant is a form of retelling: the cycle of the Seasons and the Twice-Told structure of the fantasy story are different ways of expressing the conviction what counts, what is true, is to be found again. For the medieval mind, and for the fantasy writer or reader, to say "Once upon a time" is to seize the day. [JC]