As others have pointed out, there are two questions that need answering: "What time is it where I'm not?" and "Is this a reasonable time for activity N where I'

As others have pointed out, there are two questions that need answering: "What time is it where I'm not?" and "Is this a reasonable time for activity N where I'm not?" If we drop timezones, then to find out whether N is areasonable time for (say) a meeting a quarter of the world over, you need to look up either sunrise/set times or standard business hours (which are different everywhere anyway) and judge by that. Now, you just have to look up the time difference to you.