In my office, we can start between 7:00 and 9:00, although one person got an exception for 6:00. I'm usually in between 8:30 and 9:00 and stay until between 5:30 and 6:00. For me to get there by 7:00, I would be going to bed around 10:00, which is too early for me. I'd rather stay up late. I'm single so I don't have to worry about being home early for dinner with family. Traffic is better too around those times. I also work with people across the country, so later works better.