Task: Question Understanding
Input (English): Context : The Buk missile system (Russian: ``Бук''; ``beech'' (tree), /bʊk/) is a family of self-propelled, medium-range surface-to-air missile systems developed by the Soviet Union and its successor state, the Russian Federation, and designed to fight cruise missiles, smart bombs, fixed- and rotary-wing aircraft, and unmanned aerial vehicles. 
Question : What was the year that Buk missile system entered service?
Output (English): service entry