In the National Center for Digitalization, for a few years, we have been creating a virtual library. It contains some works in mathematical disciplines, corresponding to our country. In this paper, we present digitization of the book The Eternal Calendar written by Zacharius Orphelin owned by the Library of Serbian Academy of Art and Science. Zacharius Orphelin is one of the most important Serbian writers of the eighteenth century. He is seen like a founder of the modern Serbian literature. The Eternal Calendar is our first book in modern astronomy, so it could be important for scientists interested in history of astronomy. Digitization of this book is done at the Mathematical Institute of Serbian Academy of Art and Science and it will be published in the Virtual Library of NCD .