In "Half-Life," what incident early in the story demonstrates how much time David has begun losing to his drug use?

When David wakes up in the silo, he does not remember the previous evening at all, but he does remember that he is supposed to go visit his grandmother. Thinking that it is early, before nine a.m., he hurries home. But once there, he finds that it is actually three-thirty in the afternoon. That evening, when he and Fellis return to the silo, he sees the litter they have left on previous visits, and suddenly he wonders how long they have been coming to the silo.