Which calendar?

In order to properly record a date from a document, you might need to know some geographical history.

My paternal immigrant ancestors came from four different towns in the late 19th century.

• The Cruvants came from Cekiske, Lithuania (near Kruvandai)
• The Dudelczaks likely came from either Alexandria or Zhitomir, Volhynia
• The Newmarks came from Warka, Poland (near Warsaw)
• The Blatts came from Losice, Poland

What do all these communities have in common?

All were part of the Polish-Lithuanian Commonwealth, however, by the late 19th century, at the time my ancestors left, they were all part of the Russian Empire.

This is a crucial piece of knowledge for a genealogist. Why?

The Polish-Lithuanian Commonwealth was an early 16th century adopter of the Gregorian Calendar, however, the Russian Empire didn't switch to the Gregorian Calendar until 1918. So when the territory was under Russian control, the Julian calendar was used for official record keeping, and likely for everyday use.

This means the civil calendar (non-religious calendar) all of my paternal immigrant ancestors were used to, prior to immigration, was likely the Julian Calendar. So, after arriving in America, when my great grandparents wrote down on a document what their date of birth was - were they using a Julian or Gregorian date?

Your guess is as good as mine. It's possible that they made the 12-day adjustment when they immigrated to America. Or perhaps they didn't. I have to record both possible dates of birth.

My maternal ancestors immigrated from Transylvania, Germany, Holland, and England. The first three areas adopted the Gregorian calendar in the 16th century, and Great Britain and her colonies adopted it in 1752. I have several maternal ancestors who immigrated to America prior to 1752, and when I see dates for them, if the source doesn't specify, I can't be sure if they're Julian or Gregorian.

Steve Morse's website, popular among genealogists, has a Julian-Gregorian converter, and provides years each nation changed. However, one has to be careful with his table of years. For example, he says Poland went to the Gregorian Calendar in the 16th century, which is accurate. However, once it became Congress Poland in 1815, it was under Russian control. Documents would often have Julian or both Julian and Gregorian dates (source). He says Lithuania changed to Gregorian in 1918, which is also true, but between 1586 and 1800, it was Gregorian as well. I'm unsure if the Polish-Lithuanian Commonwealth is the only example of territory which switched back and forth.