Passage: After the dissolution of the Soviet Union, Ukraine inherited a 780,000-man military force on its territory, equipped with the third-largest nuclear weapons arsenal in the world. In May 1992, Ukraine signed the Lisbon Protocol in which the country agreed to give up all nuclear weapons to Russia for disposal and to join the Nuclear Non-Proliferation Treaty as a non-nuclear weapon state. Ukraine ratified the treaty in 1994, and by 1996 the country became free of nuclear weapons.Ukraine took consistent steps toward reduction of conventional weapons. It signed the Treaty on Conventional Armed Forces in Europe, which called for reduction of tanks, artillery, and armoured vehicles (army forces were reduced to 300,000). The country plans to convert the current conscript-based military into a professional volunteer military.

Question: when was the Ukraine Lisbon protocol signed?
May 1992