Why is Robert Neville forced to stay near his home on cloudy days?

In Chapter One, we are told that Robert Neville is forced to stay near his home on cloudy days because he can never accurately calculate the approach of nightfall on those days. Although we are not told at this point in the chapter why it is so important for Robert to ascertain the exact time of nightfall, the chapter provides us with some clues as we read.

First of all, we learn that Robert has to barricade all his windows with secured planks; during daylight hours, he has to replace the older garlands of garlic at his windows with fresh ones. In the chapter, we are told that 'garlic always worked' and that Robert grows mainly garlic in his hothouse, which also has to be reinforced by planks. Later, Robert lathes thick doweling into nine-inch sticks in order to form stakes or crucifixes. We are told that, as soon as the light is gone from the day, the 'filthy bastards' will be at the house again. At this point, all the signs point to a town ruled by vampires at night.

Robert stays safe by barricading himself in his house during the night hours; this is why it is so important for Robert to watch the sky diligently as he works outside. However, on cloudy days, it is difficult to rely exclusively on the sky for confirmation of nightfall. On those days, Robert has to stay close to the house so that he is not without shelter when night approaches.