Relief's birth is just an estimate (from census records). Cinderella died at the age of nineteen. If Cinderella was born in 1823, then that would place her time of death in 1842. In truth, Cinderella and Relief could have died/been born between 1842-1844.
Here's a link if it helps: http://126.96.36.199/image_hosting/web_pages/genimages1/books/zacheus-patterson-descendants/pages-177-220.pdf