It could be that they simply don't have enough people on the job, and there's too much in the role you're doing for one person to get through in a day. That said, I find that I'll often get engaged with something at work, not notice the time, and end up leaving somewhat late. This is more that I'm enjoying it at a given moment, than it is to do with that extra time being strictly necessary. I think this is something that you need to ask yourself about. Is more being asked of you than you can get done in your contract hours? If so, why? Do you really enjoy what you do, and spend time trying to do it as well as possible?
On the time zones thing, they are a nightmare. Sometimes you need to have a conversation with someone who wakes up not long before you want to head home, and someone's going to have to take the awkward time (or more usually, a compromise must be reached). There's no perfect answer to this, since someone is always going to have to be available at a time they rather wouldn't. I think the most important thing is to ensure it's at the least inconvenient time for all parties involved.