The Collaborative International Dictionary of English v.0.48:

MST \MST\ n.
   Mountain Standard Time, the time of the 105th meridian, used
   in the mountain states of the U.S. [abbr.]

   Syn: Mountain Time, Mountain Standard Time.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

MST
    n 1: standard time in the 7th time zone west of Greenwich,
         reckoned at the 105th meridian west; used in the mountain
         states of the United States [syn: Mountain Time,
         Mountain Standard Time, MST]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

MST
       Mountain Standard Time [-0700] (TZ, MDT, USA)