GMT Differential (aka UTC Differential)

GMT stands for Greenwich Mean Time, also Universal Time. Every Time Zone is assigned a number, depending on how far they are from Greenwich Mean Time to the East or West. East of Greenwich is positive numbers; West of Greenwich is negative numbers.

For example:
London, England = 0.0 DST = +1.0

England observes Daylight Savings Time. Greenwich doesn't move; it's still -0-, so London becomes an hour ahead of that.

US TIME ZONES / GMT/UTC Differentials
Eastern Standard Time = -5.0 DST = -4.0
Central Standard Time = -6.0 DST = -5.0
Mountain Standard Time = -7.0 DST = -6.0
Pacific Standard Time = -8.0 DST = -7.0

Exceptions:
If a location does NOT observe Daylight Savings Time, then it will never change its GMT Differential

Example:
Arizona is ALWAYS -7.0. the exceptions being Arizona (except for the Navajo, who do observe daylight saving time on tribal lands), Hawaii, and the overseas territories of American Samoa, Guam, the Northern Mariana Islands, Puerto Rico, and the United States Virgin Islands.

Generally a customer will tell you immediately if they are in a State that does not observe Daylight Savings Time.