It's unofficially the same as MT, MST, and MDT. Someone is trying to abbreviate "Mountain Time" in the US. Officially it's referred to as "MT=Mountain Tim", "MST=Mountain Standard Time", or "MDT=Mountain Daylight Time".

Standard time vs. Daylight time refers to the months when the clocks shift an hour in the US. In the Spring (Mar 12 this year) most of the US moves the time forward 1 hour. This time remains until Nov 5 (this year) when most of the people in the US move the clocks back an hour. It's an antiquated rule that was originally set to allow farmers more time to work in their fields near harvest time.

Today MDT is -7 hours from UTC. On Nov. 6th of this year MDT will be -6 from UTC.