DST shouldn't be an issue unless you are working across timezones, e.g. if you wanted to know the number of work hours between midnight in Los Angeles and midnight in Sydney. If you are calculating work hours between any two business days then you will always use the standard 7.5 or 8 hours (depending on your business) as daylight savings takes effect at 3am which doesn't affect normal business hours.
In other words: think about your problem more carefully..