The boosts are only good for 1 hour, whether it is for breeding, evolving, farming, etc... Whatever one you choose will count down that many times for that 1 hour only. In your case, it will take off 5 hours in that 1 hour time period, since you used a 5x boost. So, you gain 6 total hours (5x boost, plus the 1 hour), so your Diamond will be ready 34 hours after he hit your nest...