I stopped reading the replies after the first page because people are making too many assumptions
There are no official operation hours because I don't work in IT helpdesk like you guys might. I work at a consulting company and I go project by project to different client sites. It obviously depends on the project, however I've worked on many consulting projects in the past at consulting firms, and 9AM seems to be the right time to stroll into work. I work past 6 on most nights anyway.
The read my post again. Consultants are not usually flexible in terms of start time. I mean, yeah, you can show up 30 minutes late and leave 30 minutes later, but you cannot show up at 9 AM when open of business is 8 AM. In client based consulting, it's unacceptable.
Oh, and guess what, I am a consultant also, so do not act like what I say does not apply. As a consultant, you are paid for your time. If your time includes being on work on time, then that is part of your job. As a consultant, you clients are internal as well as external, so showing up to work late while coworkers show up earlier than you is a bad thing.