Simple logic - but surprise! Case rule dates are useless

I didn't know but thought it was a great idea. I created a custom field on my case form called 'respondbydate' and 'respondbytime'. Made them date and time fields respectively. We offer two support plans, one 24 x7 and the other 12 x 5 (excluding holidays). Case rules such as duration (hours) since last modified work fine for 24 x 7 since you would check against actual hours to determine if a case should be escalated. However, 12 x5 (excluding holidays) is much more complicated since you need the duration hours to reflect BUSINESS hours not actual hours and you