Comment by bitexploder

This is addressed in Cal's book. He acknowledges no plan will survive the day perfectly. Basically, you just re-prioritize your blocks throughout the day. The goal is not to break the day into hour long blocks as much as it is to always have a priority and to maintain focus on that priority. Things happen and no plan survives contact with reality. I have found this method to work well and fit with a modern knowledge worker's work patterns for the most part.