From the Introduction: "The United States and Russia signed a new strategic arms reduction treaty--known as New START--on April 8, 2010. This treaty replaced the 1991 Strategic Arms Reductions Treaty (START), which expired, after 15 years of implementation, on December 5, 2009. The U.S. Senate provided its advice and consent to ratification of New START on December 22, 2010, by a vote of 71-26. The Russian parliament, with both the Duma and Federation Council voting, did so on January 25 and January 26, 2011. The treaty entered into force on February 5, 2011, after Secretary of State Clinton and Foreign Minister Lavrov exchanged the instruments of ratification. New START superseded the 2002 Strategic Offensive Reductions Treaty (known as the Moscow Treaty), which then lapsed in 2012. New START provided the parties with seven years to reduce their forces. Both parties completed their required reductions by February 5, 2018. New START will expire on February 5, 2021, 10 years after it entered into force, unless the United States and Russia agree to extend it for no more than five years."
CRS Report for Congress, R41219
Congressional Research Service: https://crsreports.congress.gov/