Time of sunrise various much more than an hour once you are far enough away from the equator. For example, http://aa.usno.navy.mil/data/docs/RS_OneYear.php gives over 2 hours for Texas, over 3 hours for New York and almost 4 hours for Washington.
Yes, hence the "approximately." The current system is a compromise; you can't go slewing the time continuously across latitudes and day of year, at least not yet, but you can't shock people with three hour jumps, either. I for one would welcome two dimensional time zones and continuous slewing. In an era of smart timekeeping, anything is possible.
