@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. 

 @3c8652e3 
Yes, the history is long, it doesn’t start in 1949.