I like core hours in theory, especially if the window is that small (in practice I've seen it more like 10-4). What should the window be when the team is split

I like core hours in theory, especially if the window is that small (in practice I've seen it more like 10-4). What should the window be when the team is split across timezones like PST & EST, with a 3 hour difference? Either someone is online very early, or very late.