In the 1772 First Partition of Poland, the Prussian king Frederick the Great annexed neighboring Royal Prussia, i.e. the Polish voivodeships of Pomerania (Gdańsk Pomerania or Pomerelia), Malbork, Chełmno and the Prince-Bishopric of Warmia, thereby bridging the "Polish Corridor" between his Prussian and Farther Pomeranian lands and cutting remaining Poland off the Baltic Coast. The territory of Warmia was incorporated into the lands of former Ducal Prussia, which, by administrative deed of 31 January 1773 were named East Prussia. The former Polish Pomerelian lands beyond the Vistula River together with Malbork and Chełmno Land formed the Province of West Prussia with its capital at Marienwerder (Kwidzyn). The Polish Partition Sejm ratified the cession on 30 September 1773, whereafter Frederick officially went on to call himself a King "of" Prussia.
In what year was the territory of Warmia incorporated? 
31 January 1773