Mountain Standard Time (MST) is a time zone that is 7 hours behind Coordinated Universal Time (UTC-7). It is primarily observed in the western United States, parts of Canada, and Mexico. MST is used during the winter months when Daylight Saving Time is not in effect. During Daylight Saving Time, which typically runs from the second Sunday in March to the first Sunday in November, regions observing MST switch to Mountain Daylight Time (MDT), which is UTC-6. This adjustment helps to make better use of daylight during the longer days of summer.