Billable hours are much better than plain ol' non-billable hours. They're also another example of mutable time. For example, for a psychiatrist, a billable hour is about 45 minutes (i.e. the time you might spend on the couch) whereas for an engineer in a pre-IPO company, a billable hour might be 80 or 90 minutes (because of the restrictive laws regarding overtime for W2 workers).