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 ) .