The book is devoted to the origins of the modern civil calendar. How many years have passed since the birth of Christ? Was it not in vain that the beginning of the third millennium was celebrated in 2001? Which calendar is correct - the “old” one, used in Russia until 1917 and still used in the Russian Orthodox Church, or the “new” Western European one? What is the “solar calendar” and what is the “true length of the solar year”? Why do Orthodox and Catholics celebrate Easter sometimes on different days, and sometimes at the same time? The reader will find the answer to all these and many other questions in this book. The book does not require special knowledge from the reader and is intended for everyone who wants to understand our history and its many mysteries.