She didn't know what time she had come through. It had been a quarter to eleven when she came out of prep—she had seen Ms. Montoya looking at her digital and asked her what time it was—but she had no idea how long it had taken after that. It had seemed like hours.

The drop had been scheduled for noon. If she had come through on time and Probability was right about the slippage, it would be six o'clock in the evening, which was too late for vespers. And if it were vespers, why did the bell go on tolling?

It could be tolling for mass, or for a funeral or a wedding. Bells had rung almost constantly in the Middle Ages—to warn of invasions or fires, to help a lost child find its way back to the village, even to ward off thunderstorms. It could be ringing for any reason at all.

[…]

She was beginning to feel stiff with cold though she had been lying there only a little while. Whatever was poking her in the side felt like it had gone through her rib cage and was puncturing her lung. Mr. Gilchrist had told her to lie there for several minutes and then stagger to her feet, as if coming out of unconsciousness.

Basic set up:

Kivrin Engle, the heroine of Willis' novel, has just travelled back in time to 14th century Oxford, England. Here she is trying to figure out what time of day she's been dropped into.

In sci-fi, characters don't just travel to the future. They can also travel to the past. In Willis' Doomsday Book, the protagonist does, indeed, travel to the past: way back the Middle Ages, to be exact.

Here we see her in the first moments after her journey to the past, and she's trying to figure out exactly what time of day it is. We see her disorientation and confusion. Wouldn't we be disoriented and confused if we'd skipped over a few centuries in a matter of hours?

The narrator signals to us that we are in a new time by giving us details that reflect that new time and age. Kivrin thinks that it's too late for vespers—an evening prayer that takes place in churches, and it was very common in the Middle Ages.

The constantly ringing bells are another sign that we've popped back into the Middle Ages. As the narrator notes, "bells had rung almost constantly in the Middle Ages—to warn of invasions or fires, to help a lost child find its way back to the village, even to ward off thunderstorms." Through these physical and sensory details, the narrator transports us readers, along with the protagonist, back to a different age and time.