The Mountain Time Zone of North America keeps time by subtracting seven hours from Greenwich Mean Time, during the shortest days of autumn and winter (UTC-7), and by subtracting six hours during daylight saving time in the spring, summer, and early autumn (UTC-6). The clock time in this zone is based on the mean solar time at the 105th meridian west of the Greenwich Observatory. In the United States, the exact specification for the location of time zones and the dividing lines between zones is set forth in the Code of Federal Regulations at 49 CFR 71.