MDT refers to Mountain Daylight Time, while MST refers to Mountain Standard Time. Mountain Daylight Time is 6 hours behind Coordinated Universal Time (UTC), whereas Mountain Standard Time is 7 hours behind UTC.
MDT is sometimes listed as UTC - 6 hours or -0600, and MST can be listed as UTC - 7 hours or -0700. Many western states of the United States follow Mountain Standard Time during the winter and Mountain Daylight Time during the summer. In some states, parts of the state follows this pattern, but other parts do not. MDT and MST are also used in parts of Canada's and Mexico's western mountainous regions.