You still have to know a bit about your buddy, though. You can probably assume he'll be awake at his midday, but what about 7:00? Is he an early riser? Or does he have other commitments at that time? In reality you'll just learn you can call around 16:00 your time. At that point it's just a number and doesn't matter what it is.

If things really were more complicated and you always needed to ask the question of "when can I call today", would it really be that difficult to incorporate schedules into contact records so your phone can tell you "available after 23 today".