@adbb894e During the period of the Partitions of Poland, Russification of so-called Congress Poland began after the Decembrist uprising in 1825, but kicked into high gear in the late 1860s.  Vital records, which had been kept in Polish using the Gregorian calendar, were mandated to be kept in Russian after 1868, and the records had to reflect the Julian date as well as the Gregorian. 

 @c043ccf2 Hence the joke in Dr. Strangelove: President Merkin Muffley, played by Peter Sellers.