In the US the start and end of DST is encoded in law. So it doesn’t change much (and might become moot if the House ever decides to vote on the bill passed by the Senate to stick to DST full time). So the easiest thing to do would be to create a rule that uses a cron trigger at 3 AM on March 12th and November 6th and update the variable as needed.
March 12th cron: 0 0 3 12 3 ? *
Nov 6th cron: 0 0 3 6 11 ? *
You could also put these into a DateTime Item and use a Time is <Item> to trigger the rule. Then you can just populate the Items once and let restoreOnStartup handle the rest.
The rule can be super simple and check to see if the alarm is 6 and change it to 5 and vice versa, or check to see what month it is when the rule triggers and set it as appropriate.