I live in Europe, while my clients are mostly from US. Thus, the smallest timezone difference between me and my clients is -4hours, the biggest being -9hours. Yet, I also have a customer in Asia, giving me a +4hours customer. Scheduling calls starts to be a bit painful.
My method is to have 2 days a week marked as call-possible-days, and on those days I offer time slots when a client asks for a call or I need one.
Do you have any other tools/methods you use? This is not a "wood under the toe nail" problem, but definitely something that grinds my gears.