Each day of the year the Earth turns around its axis of rotation, to give the face or the back to the sun, which shines its light more or less hours depending on the latitude or angular distance from Ecuador, as the circles of latitud or parallels.
The 360° of the celestial sphere is divided into 24 time zones of 15 degrees each and determine the solar time for each of the main 24 lines of longitude o meridians.
Time zones are defined by national borders, so it does not match solar time, and less in some countries when their clocks ahead one hour in daylight saving time.
It's enough confusing to know the current time or hour in any place on Earth for a traveler who moves through different time zones, to arrange a videoconference between countries or to find out what the next working round in a multinational company which designs in a meridian, verify 8 hours later and write documents next 8 hours in another country, in continuing cycles of 24 hours without rest, in which staff are always awake, as in the Empire on which the Sun never sets.
On the Web there are many world clocks and maps with time zones as 24timezones, worldtimezone, mytimezonemap o timeticker, but there aren't good public use programs for synchronized times, so I've been doing an Excel spreadsheet to tell the time anywhere in the world.
You can download it from the AyudaExcel.com forum if you are previously registered in less than a minute: