Where I worked, flex time meant that you could work longer on a particular day to take some extra time off by working a shorter day. To keep things in check, each two-week period had to work out so you'd worked 80-hours. This was to prevent you from accumulating a big quantity of hours off if you worked long hours for too long without taking time off. If you had a project that was expecting more than 40-hours/week on a long-term basis, then overtime was approved. This can get messy depending on the labor laws in your state as a week with more than 40-hours might require you to pay overtime, even if it would average out to 40hours over time where flex time might have made everyone happy.