You should be aware that all calendars are not created equal. Throughout history the calendar has been changed, adjusted and tweaked. The calendar we use today is called the Gregorian calendar and was introduced by Pope Gregory XIII in about 1582. This new calendar was a big change from the dominant Julian calendar used at the time.The Julian calendar was introduced by Julius Caesar in about 48 BC and was used by many countries into the 20th Century. Many branches of the Orthodox Church still use the Julian calendar for religious events.
Some other calendars used in history are the French, Jewish, Indian, Chinese & Islamic Calendar. Whenever there was a change in the use of a calendar, the change was not always smooth and sometimes the conversion was done over time to add or subtract dates gradually.
Understanding calendar usage is important to know when doing genealogical research because you need to know when and where each calendar was used in recording dates of events for your ancestors.
In the case of the area of calendars in Poland, prior to the partitioning of Poland in 1772, the Gregorian calendar was used, but in the areas that became part of Russia, the Julian calendar was again implemented up until after WW1. Most of the time the priests recorded dates in the record in both Julian and Gregorian calendars. A person must know which date was which. Some priests were rebellious to the government and did not write both dates. If only one date was recorded, there is no way to be sure which calendar was used. Poland’s history is confusing enough in tracking ancestors, but then to add the difference in calendar usage just makes it all that more interesting.