In 1994, the NRRC assumed responsibility for the exchange of quarterly notifications for Phase II of the Bilateral Verification Experiment and Data Exchange, as well as the Agreement on Reciprocal Advance Notice of Major Strategic Exercises, which requires each side to notify the other no later than 14 days in advance of any of its major strategic exercises involving heavy bombers to be held during that calendar year (agreed in the Wyoming MOU of 1989). In 1994, Presidents Bill Clinton and Boris Yeltsin also agreed to have their respective national centers exchange quarterly notifications containing descriptions of current levels of strategic offensive arms and their planned deactivation and/or elimination (the SDX). For fifteen years, the SDX data exchange provided a valuable supplement to the START Treaty. The SDX process was discontinued with the advent of the New START Treaty.