instruction: Given a reference text about NATO, when did Finland become a member?
context: The Republic of Finland has been a member of the North Atlantic Treaty Organization (NATO) since 4 April 2023. Finland had formal relations with NATO since 1994, when it joined the Partnership for Peace program, and has been a member of the European Union (EU), which largely overlaps with NATO in membership, since 1995.
response: April 4, 2023