Do you like to travel with the finger on a world map? Or are you just thinking of booking a spontaneous last minute trip? Also, you might want to phone your aunt in Australia. Then it would be good to know the local time at the other end of the world. On the map (which can be enlarged with a click), you find the rough division of the world into time zones. More detailed information can be obtained from the Overview of Time Zones which has been compiled by the time experts of PTB.
By the way: Did you know that less than a hundred years ago, there were some odd regulations in Europe? Persons travelling in 1928 from Germany to the Olympic Games in Amsterdam had to set their watch forward by 20 minutes. Travelling to Holland in winter, however, meant that you had to turn your clock back by 40 minutes.