The way it works is that when you select UTC, Slackware writes a file in /etc called localtime containing the word "UTC." It checks this on startup and shutdown and invokes hwclock with a special flag to let it know that it is set to UTC and makes it act appropriately. The easiest way to switch back and forth, is to type "timeconfig" as root. This will repeat the setting of your timezone and you can then pick UTC. You'll still have to select a local time, but the clock should first be set to UTC to get it to show up right.
