When did Finland become a Russian grand duchy?

Finland became an autonomous grand duchy under the Russian throne in 1809.12

Following the Russo-Swedish War of 1808–09, the Russian emperor Alexander I offered to recognize Finland's constitutional developments and grant it autonomy. The political framework for this status was established during the Porvoo Diet in 1809, where Finland was recognized as a united political entity. Although Finland remained formally part of Sweden until the Treaty of Hamina was signed later that year, the 1809 Diet marked the beginning of its transition to a grand duchy under Russian protection.123