Here's a map I did of the hypothetical time zones of the United States, showing the time relative to when I started doing the clocks.
You could simplify it by using states rather than counties:
GMT-4.5: DC, Delaware, Maryland, New England, New Jersey, eastern New York and Pennsylvania
GMT-5: the Carolinas, Florida, Georgia, Ohio, Virginia, West Virginia, western New York and Pennsylvania
GMT-5.5: Alabama, Illinois, Indiana, Kentucky, Louisiana, Michigan, Mississippi, Tennessee, Wisconsin
GMT-6: Arkansas, Iowa, Kansas, Minnesota, Missouri, Nebraska, Oklahoma, eastern Texas
GMT-6.5: Colorado, the Dakotas, western Texas
GMT-7: Montana, New Mexico, Utah, Wyoming
GMT-7.5: Arizona, Nevada, Idaho, eastern Washington
GMT-8: California, Oregon, western Washington
The division takes into account population density, which is why some states are not where you might expect them to be. In the end, I think using one-hour increments is far more practical.