The Canterbury Tales constantly mark the passing of time. The narrator often tells us exactly what time it was when a particular event occurred, and even the way he (or the Host) arrived at this calculation by coordinating the day of the year with the position of the sun. The Host seems to have a sense of urgency about the tale-telling competition, constantly reminding the pilgrims that time is slipping away from them. When he waxes poetic about time, the Host compares it to a stream that's running quickly, the water never to be regained. Yet, despite this fatalism, or sense of powerlessness, about the passage of time, the poem also suggests that it's possible to avoid what's depressing about lost time by using it well. That's probably the reason the Host is so emphatic that the pilgrims keep on telling those tales. To him, at least, tale-telling is a way of using time well.
The frame story of The Canterbury Tales marks the passing of time in order to make its readers understand the urgency of reaching salvation, represented by Canterbury.
The language used to talk about time in The Canterbury Tales emphasizes the way in which time is always escaping from us, despite our best efforts to prevent that from happening.