Just a hunch, but I believe Casey is the one who wrote the '03 and the 'Carpe Diem' the same time as the June 21 (presumably '08) entry. I also think she tore out the pages before June 21, as this was an entire new chapter in her life and she was starting with a fresh slate. The writing style on the front cover was similar to writing she used in jail with Cookie (or whomever) and the red ink was also a give away. I believe she wrote the '03 and the purposely ambiguous June 21 entry thinking she was cleverly fooling whomever might pick up the diary. IMO.
ETA: It makes sense to me that she had the diary with her at Tony's and it was sent home with the rest of her belongings by Lee.
ETA: It makes sense to me that she had the diary with her at Tony's and it was sent home with the rest of her belongings by Lee.