Mountain Daylight Time (MDT) is a time zone that is observed in parts of the western United States and Canada during the summer months. It is 6 hours behind Coordinated Universal Time (UTC-6) and is used from the second Sunday in March until the first Sunday in November. During this period, clocks are set forward one hour to make better use of daylight in the evenings. MDT is part of the larger Mountain Time Zone, which also includes Mountain Standard Time (MST) when daylight saving time is not in effect.