The issue of changing the Julian calendar to the Gregorian calendar in Ducal Prussia has not been discussed separately so far. The problem seems simple, but it has caused quite some discussion in the literature to date. In his seminal work on the auxiliary sciences of history, Józef Szymanski provides information about changing the calendar, but with the wrong date, namely 1610. The publisher of Max Toeppen’s work, The History of Masuria, gives the date of 1700, when the Gregorian calendar was actually introduced, although in German Protestant countries. At the same time, Toeppen himself was not interested in this problem and he gave the dates from the documents, without changing them to the Gregorian calendar. This was what most German researchers subsequently did, and following them Polish researchers. However, this means that the dates between 1582 and 1612 are generally given according to the Julian calendar and many of them need to be verified. One such example is dating the location of the town of Lec (Giżycko) to May 1612.