The sun does not shine with the same intensity over all parts of the world at a particular time. If it is overhead at one place (midday), then it does not shine at all (midnight) at the place directly opposite to that place on the earth. So if we have the same time, then 12 noon will mean midday at one place, and midnight at another. To avoid such an awkward system, we have the system of time zones, so that everywhere a particular time means the same stage of the day.
