This is foreshadowing. As Eddie passes, he is searching for the phrase that fits the calendar, but can't think of it. Then, Pop Harper uses the phrase about an old, technologically obsolete typewriter, but Eddie lost the connection.
"Your days are numbered."
Nice touch. That calendar plays a vital role in the story, not just because it tracks the dates, but because of what d’Anconia does to it later in the book.
Very good!
It's been over two decades since I read this book and I was too young and silly at the time to really get it. Reading it now I realize I don't even remember much about it other than some character names and the general idea - it's like reading it for the first time. Except I do remember how it ends, but this time I can savor getting there. This is fun!