Time Zones in United States.
The Mountain Time Zone refers to time zone which observes time where seven hours are subtracted from GMT (UTC/GMT -7). This is called Mountain Standard Time (MST). The clock time in this zone is based on the mean solar time of the 105th meridian west of the Greenwich Observatory.
Mountain Daylight Time (MDT) refers to time where six hours are subtracted from GMT (UTC/GMT -6). This time is used during the daylight saving time, which is observed throughout the sunnier months of the year, during the spring, summer and autumn.
In the United States and Canada, Mountain Time Zone is generically called Mountain Time (MT). Specifically, it is Mountain Standard Time during winter, and Mountain Daylight Time when daylight saving time is observed. The name of the time zone refers to the fact that the Rocky Mountains, which range from northwestern Canada to the U.S. state of New Mexico, are located almost entirely in this time zone.
Effective since 2007, the local time changes from MST 02:00 to MDT 03:00 on the second Sunday in March and returns to MST at 02:00 MDT to 01:00 MST on the first Sunday in November.
Most of Arizona does not observe daylight saving time, and during the spring, summer, and autumn months, it is on the same time as Pacific Daylight Time, though it is still called Mountain Standard Time in Arizona. The Navajo Nation, most of which lies within Arizona, does observe daylight saving time, although the Hopi Nation, as well as some Arizona state offices lying within the Navajo Nation, do not.
Greenwich Mean Time (GMT) is originally referring to mean solar time at the Royal Observatory in Greenwich, London. GMT and Coordinated Universal Time (UTC) can be seen equivalent when fractions of a second are not important.