During Nicholas’s reign, Russia used the old-style Julian calendar to record dates, when most of the rest of the world was using the new-style Gregorian calendar. This meant Russian dates lagged twelve days behind during the nineteenth century, and thirteen days behind during the twentieth. Thus Anastasia was born June 5, 1901, according to the Julian calendar, but June 18, 1901, by the Gregorian calendar. Since readers of The Family Romanov are familiar with the Gregorian calendar, all the dates in this book are given in the new style.
From Before You Begin in The Family Romanov