Original: The Mountain Time Zone of North America keeps time by subtracting seven hours from Coordinated Universal Time, sometimes called Greenwich Mean Time (UTC-7) during the shortest days of autumn and winter, and by subtracting six hours during daylight saving time in the spring, summer, and early autumn (UTC-6).
Simplified: The Mountain Time Zone subtracts seven hours from UTC during standard time (UTC-7) and six hours during daylight saving time (UTC-6).