Actually, I regularly get in around 6, take a long lunch to hang out with my family and then leave around 4.
I think a couple of things help the culture of this at GitHub - one being that all of the directors keep wildly different hours, which I think is a helpful example. Not only do I generally leave around 4, I try to say 'later guys' to let everyone know that I am. If the bosses at your job are working 8-6 then that tends to trickle down. If they set the example, then everyone else can feel comfortable in doing so as well.
I think a couple of things help the culture of this at GitHub - one being that all of the directors keep wildly different hours, which I think is a helpful example. Not only do I generally leave around 4, I try to say 'later guys' to let everyone know that I am. If the bosses at your job are working 8-6 then that tends to trickle down. If they set the example, then everyone else can feel comfortable in doing so as well.