It should tell you something that I've been at MIT for 20 years and I have no idea. Faculty at these places are free to set their own hours but are also workaholics. We all work far more than 40 hours per week on average, and as a result nobody cares what we do in any particular week.

MIT pays us a salary for 9 months of work during the academic year. If we want to draw a salary for the summer, we need to pay for it out of our research grants. And if we want to draw a full 3 months of summer salary, we need to commit that we will not take vacation during that time.