In 2000, the Novgorod archeological expedition under the direction of academician V.L. Yanin discovered the oldest Russian book, which dates from the first quarter of the 11th century. The book consists of several wooden plates for making wax tablets with inscriptions. Besides the main text containing psalms, edges and other parts of the plates include additional, barely visible inscriptions, the decoding of which was recently published by academician A. A. Zalizniak. The texts contain some mathematical inscriptions - the so-called “number alphabets” used for teaching numbers in a system written with letters. They also mention several times the year 6507 (999 AD), in which the monk Isaakii received the title of priest in Suzdal and became hieromonk. The inscriptions were made either by Isaakii himself (the most likely interpretation, according to Zalizniak) or by a professional scribe. One can conclude that the knowledge of arithmetic was used in ancient Rus at the end of the 10thh century (999) for chronology, among other purposes. Hieromonk Isaakii can be considered in this regard a predecessor of Kirik of Novgorod, the earliest Russian mathematician known by name heretofore, who authored the mathematical and chronological treatise of 1136.